I do not take this as lightly as others do. These cars are very flexible and with lots of rust and panels missing yes they do move around. I myself would have taken to a frame shop and had it check out to see if it was square to start with. These cars are very flexible.
Do a search on the site and I have posted the chassis measurements from the Ford manual a couple times before.
Any twist or incorrect position of a panel will translate into fit issues in the future. These were held to about 1.0 mm at the factory and I doubt that you will be able to do that without a jig to position the body/chassis correctly.
Without a frame every piece of the underbody including the quarters and roof are a structural part.
Doors need to be in place to check gaps before welding it all back together also. You need a really level floor so you can measure everything.
I know some got lucky and things fit but I also see some of the horror stories on youtube where nothing fits.
I went and looked at a car that had floors, trunk, cowl and quarters replaced. They did not do it right and it was more or less a parts car would never have looked right or made a good car.
Is this car something special? Why would you not look for a better chassis to start with. I have seen almost rust free 6 cylinder cars go for a few hundred dollars on craigslist. I myself have parted out cars with less rust. When you replace all those panels the value of the car goes to a resto mod without original metal with factory date codes.
My opinion and others will have theirs.
One of our members has quite a few videos on replacing panels do a search on here and youtube.
You offer some good advice, but don't be mistaken - none of this was "taken so lightly," as you say.
Checking it for proper frame alignment is something you can do yourself with the correct information at your disposal. QCode351Mach provided me with factory chassis drawings, and the measurements were pretty close... before I took anything apart. I measured, and measured, and measured before taking any steps. Then measured, and measured, and measured again along the way... followed up by more measuring once done. The car is dead on as outlined on the chassis drawings, which would've been 'just fine' if they were within 3/16" tolerance, again, according to the factory chassis data.
As bad off as my car was, I literally had nothing to lose, so I replaced the front clip. Then the floor pans, then cowl pieces, then the trunk, taillight panel, quarters, and everything else that needed to be done. I did not simply 'get lucky,' either.
Was it a special car for me? Not particularly, unless you count it being the only one within 250 miles - West Texas is a LOT different than anywhere back east, where there are plenty of hiding places to find pretty much whatever you like with reasonable prices.
The person on this site you speak of is Scott, or QCode351Mach... and he knows his stuff and has been very generous in sharing his advice and encouragement with the group.
Bottom line: if you want a concourse-quality vehicle or something to sell to someone who wants a concourse-quality vehicle, then by all means find the best one you can afford and dump all your money into getting it 'done right' by a professional. If you want a nice, safe, project car that will be 'just fine' because it's not particularly valuable to anybody but you, then take a chance on yourself and have fun rebuilding your car.