ok, deep breath.
I went through this in 2007 on my car similar damage.
here are some of the pictures from the repair.
http://s1031.photobucket.com/albums/y377/72hcode/towers%20and%20rails/#!cpZZ1QQtppZZ20
lets discuss:
The rust you don't see is 100 times worse then the rust you see.
That area under the shock towers is a Nexus between the engine support and suspension support, its also a major structural point.
The truth is based on the photos above, the shock tower is toast, you can see its completely rotted across the rail.
because i can see through so much of that tower most likely the engine cradle is toast which is also the lower suspension arm mount. these parts are not reproduced. so your looking at minimal new shock tower and engine cradle(lower suspension mount). now your talking alignment of the vehicle and getting into a complex repair that requires more skill then just putting in new floor pans or patching quarters.
here is where i will disagree with OMS, based on the damage i see in photos and my experience you do not have enough good metal for a simple top patch, it needs to be determined if there is enough thick steel left on the Rail sides to even warrant repair or replacement. To go through the process of determining the extent of the damage.
1) the engine needs to come out, you need full access to the back side of the shock tower with the engine out of the way, Exhuast manifolds Cover a lot of damage.
2) things need to start getting cut out of the way, right down to the rail itself to see how badly thinned the metal is.
once at that point if more then half of the rail box in that area is good then you can sheet steel the rail and rebuild it.
if just the bottom of the rail turns out to be solid then your looking at a complete front clip if both sides are just as bad.
on my car i was VERY close to needed a full front clip. the only reason i didn't was because i found shock towers and an engine cradle right away and had the engine aprons. if i had nothing then i would of gotten a donor car.
now something this important was beyond me so i hired a frame/welder to do the work.
the problem is things spiral out of control very fast once you start ripping into one area the problems in another area become apparent.
on my car i had to rebuild the front clip all the way to the A pillars including the cowl.
cost of materials and labor was just over 5000$ then there was extras on top of that.
car took about 5 months to complete the repair since the guy was busy and worked 20 hours at a time on it.
so i could make payments on his labor.
my car required:
new towers (required additional repair to fix replacements)
new engine cradle(required addition repair to fix replacement)
new engine aprons X4
new rad support
new cowl to engine bay aprons(required additional repair to fix replacements)
patches to drivers and passenger rails.
patches to fire wall.
hand fabrication of cowl area.
hand fabrication of missing and or rusted away panels not included with reproduction panels.
once the welding work was completed he primed and painted the area to prevent rust from forming again.
I then re-striped the area for additional work, striped the firewall and cowl area down to the rockers.
removed undercoating, then reprimed and finish coated the engine bay.
i'll stop here since there are tons of details, look at the photos in my link first I have more of the original damage before the repair started so you can compare.
I will say this: damage to 71-73 cars in that area is common and usually the Death nail for the car. if you are going to resell the vehicle for profit, then get out of it now. If you really do want to fix the damage like i did it will cost a lot to bring it back, and isn't a simple repair.
i'll check back on this thread later.