I see you have had a lot of nice responses. Yet, I feel compelled to share my opinion on this matter. I think the question re: a restoration being "worth it" depends on where you associate the value with the Mustang. If you thoroughly enjoy it and just want to try to bring it back to a more original, fresher condition, then it is a "simple" matter of assigning a monetary value to the pleasure you derive. I find this is best done by looking at the cost of replacing the vintage Mustang with a new or much newer vehicle you may get the same degree of pleasure from. For me, my "Compare Other" came in the form of a 2020 Shelby GT500 we ordered, purchased, and took delivery of. Admittedly, that is a stretch to consider the 2020 Shelby to be something to compare a 73 Mach 1 to, but it was all I had to work with re: new or newer pony cars. So, my bar was set pretty high. To make things even more difficult, we have a 69 GT500 that is truly in near perfect condition that I also used to compare the amount of pleasure I take in driving these older Mustangs and Shelbys. So, I set a baseline amount I would be okay with putting forth to get our 73 Mach 1 into a highly restored condition, I figured about $30-35k or so.
Well, before I go any further let me say this... In the computer industry when coming up with estimates for building a custom solution I found a sure fire way of coming up with a solid estimate. After penciling in all the anticipated estimates for hardware, operating system, applications, networking, printer, custom programming tine, etc., double the figure you arrive at. And, once you share that doubled estimate with anyone, double it again. Maybe, just maybe with any luck at all you may just come in at 4 times the initial estimate - unless you shared the 4x figure with anyone. In that case you can double it yet again. It is just how the Computer Gods work. And there is the saying I have been peddling for a few decades. The 6 most dangerous words in business are, "All you have to do is..."
Well, it turns out this hobby of ours (or obsession) can be modeled in just about the same way. I purchased a rear trunk panel for a recently purchased 1973 Mach 1, as the original panel had some rust around the taillight lens seals. I then went to a restoration shop after I decided I really ought learn to do TIG/MIG welding on such a nice car. They had no problem with the trunk rear panel I had purchased, along with the fuel filler brace. I told them I would really like it if they could work the panel in while preserving the surrounding panels as much as possible so we could minimize having to do any paint matching on any other surfaces. I was told that ought not be a problem, so I left the Shelby in their hands to do that little bit of work.
Well, yep, as you may have guessed we got a call a few days later asking that we come on out to see some things about the Mach 1. I figured they found a few other things they wanted to take care of while the rear panel was off. I guess I was right, but I did not have a clue to what degree other stuff was found. Now at this point I ought to point out that we had looked closely at the Mach 1 before buying it, being wary of rust. We found the rust at the taillight pod seals, at the lower rear corner of the river door, and two small spots on the hood. All fairly typical areas for Mustang rust on those cars. The underside truly looked almost perfect where only one hole was found between the inner cabin and under a floor pan, and that was for a screw used to mounts a wiring conduit for some rear speakers - plenty forgivable. I was not prepared mentally for the show that was about to drop on us. But, one could not have been able to tell by my reaction.
It turns out that while peeling off other rubber parts, and removing some chrome trim around the rear window (for some other minor work), the body tech found a lot of deeply located rust, way into the crevices of some seams. Areas we could not have found unless we pulled off bright trim pieces and looked really deep. The owner asked what i would lie to do. We had hade a commitment to the prior owner to always do right by the Mach 1, and promised we would not ever sell it to some high sci told the owner to keep digging and once he was school kids who would end up getting killed in it (engine was not stock). So, in keeping with the promise made to the prior owner, and for love of the breed, I asked him to continue with replacing the damaged metal, while preserving as much as the original metal as possible. We would visit the shop once a week to see the progress, and as it turned out to also keep approving ever more money for newer discoveries. Yeah, for love of the breed...
As it turned out every crevice or seam on the car had come under attack. And the price of resectioning new metal with old metal that was not rusted is very labor intensive. And using old school lead to fill various gaps and seams like the factory did back in those days was also very costly. But, I knew what I wanted, and I was not going to roll over on this Mach 1. 13 1/2 months later, and over $135k, it was done. Yes, the final billing pretty closely followed the computer solution rule of thumb of doubling an initial estimate, then doubling that once again. So, before you make a final decision on what to do with your Mustang, I suggest you work out an estimate for the cost of the stuff you would like done, then quadruple it and tell nobody lest the pony gods come after you for yet another bite.
I do not want anyone to get the wrong idea about any regrets re: our 73 Mach 1. We have no regrets. Had I been gifted with perfect views of the future it is possible we would have passed on this Mach 1, and taken the same money (or less) and found a really nice Boss 351. But, those did not have A/C as an option, and I wanted A/C in any vintage pony cars we would wind up with. The 69 GT500 had factory A/C, but I would not have added A/C to a Boss 302/351/429, no more so than I would have added A/C to a First Generation Shelby. So, it was likely I never would have purchased a Boss 351 anyway. But for even 1/3 of what we put into our Mach 1 I could have found one that was in pristine condition. However, I figured there were reasons for what our experience was. First, we have a story to tell re: how these cars can get out of hand so quickly when problems are found. Next, we had the cash available, and frankly we barely felt the movement of that much cash going into the Mach 1. Then there is the part about me keeping my promise and doing right by the car at every turn. And finally, despite the high cost when playing the game, we ended up with a really nice car.
There are other things we did to it in addition to the restoration. I replaced the ***** light instrument panel with a Dakota VHX analog gauge kit. installed Classic Air A/C, installed a full length center console and replaced its ash tray with a Power Panel for two 12 volt outlets, a dual USB outlets, and a digital voltmeter display, installed a Garmin Drive 51 GPS with optional wireless backup camera, replaced the original cooling system with a dual electric fan cooling system, upgraded the alternator. As if that was not enough, to help adapt to the prior owner building the engine up to a decent 360 HP, and swapping the original 2.89:1 rear axe drive with a 3.5:1 TractionLok unit, we pulled the C4 auto tranny out and swapped in an AOD in order to reduce the engine RPM at highway speeds, and to recapture a lot of that top end speed with a powertrain that works against higher air resistance at higher speed levels.
Anyway, if you decide to begin modifying and upgrading the Mustang, remember to quadruple the estimates and you will do fine. have fun, either road you take...
https://youtu.be/_Fl0B-qmlLI
