Unibody rework questions

[Hack]

I'm about to start working on fixing the body on my '70 convertible. It was originally a 6 cylinder, no option car, so it isn't especially valuable as a collectible.

It's pretty much toast underneath and my plan is to use a solid '70 coupe as a replacement for most of the underbody. I will add the factory convertible reinforcements. Since this car isn't going to be original, I'm planning to fab some of my own additional reinforcements.

Question:
The torque boxes are really solid on this car, but I'm worried about future rusting. Would it be stupid of me to cut some of the boxed areas of the car open so that I can put better rust preventative on them?

Any recommendations on reinforcement strategies? I was considering some of the Boss 302 shock tower reinforcements, some kind of additional subframe connectors and potentially torque box reinforcements. I also like the idea of reinforcing the cowl, but there isn't a lot of extra room in there.

 

[2+2GT]

It's pretty much toast underneath and my plan is to use a solid '70 coupe as a replacement for most of the underbody.
This stuff is all available new, in convertible configuration, making it a lot easier to install.

I will add the factory convertible reinforcements. Since this car isn't going to be original, I'm planning to fab some of my own additional reinforcements.

Question:
The torque boxes are really solid on this car, but I'm worried about future rusting. Would it be stupid of me to cut some of the boxed areas of the car open so that I can put better rust preventative on them?
Drill holes as needed to insert the sprayer necessary to use the non-hardening 3M rust preventative.

Any recommendations on reinforcement strategies? I was considering some of the Boss 302 shock tower reinforcements,
A 1970 convertible should already have them.

some kind of additional subframe connectors,
A convertible Mustang already has stunningly heavy-duty subframe connectors, in the form of the inner rocker panels. Far stiffer and stronger than the puny tubes sold for this purpose.

and potentially torque box reinforcements.
Solid, non-rusty torque boxes provide excellent triangulation, if they are welded in properly.

I also like the idea of reinforcing the cowl, but there isn't a lot of extra room in there.
The cowl is also a big torque box connecting the "A" pillars.

Convertibles also have many other reinforcements, such as the lower seat platforms and center plate, rear seat back panel, and rear crossmember.

 

[Hack]

Thank you for the ideas about rustproofing.

The Boss 302 chassis mods I'm talking about are the reinforcements listed in the pamphlet (I think it might have been put together by Ford, I don't have it in front of me). I'm not talking about what came original on any Mustang, but what was suggested by Ford to reinforce the Boss 302 for racing. There are some cross member suggestions including removing the eccentric, and bracing for the inside of the shock tower underneath where the UCA mounts.

The problem with buying aftermarket parts to fix the convertible is that there isn't enough there, IMO. My dad bought the car for me in 1990, so it has some sentimental value. However, it was an original Minnesota car and the floors were replaced (very poorly with sheet metal screws!) before I got it. The replacement floors currently in the car are no good due to additional rusting in the 20 years that I've owned the car. All the inner frame rails and torque boxes are completely gone front to rear, the shock towers are deteriorated, the trunk area is full of rot, the rear wheel wells are constructed of plastic filler - of course the cowl and lower area of the firewall are also no good. The door pillars are really rotten. I didn't do any of the original structural work, but looking at the car it doesn't seem "repairable" to me. Basically I would be constructing an entirely new vehicle, and to me the expense is prohibitive. This car cosmetically looks ok, but I even have to buy new doors. The previous owner reconstructed the lower inside parts of the doors out of plastic filler. They are really bad!

I have a solid coupe with none of the rust problems. I could cut the torque boxes out of it and weld in the convertible style boxes. Or I could add reinforcements to what is there. I would rather not cut the originals out, because it seems to me like that would compromise the structure and it seems like a risky move.

I checked the windshield angle on the coupe and it matches the convertible. I think that there aren't too many other dimensional issues I should be worried about. My goal isn't to make something I can trick a buyer into believing it's original, but having a solid, rust-free car to drive and enjoy.

 

[MitchGT]

why cut a perfectly good car when all of the parts are available new? Sell the parts car, and use the money to buy brand new non rusty parts.

 

[Hack]

why cut a perfectly good car when all of the parts are available new? Sell the parts car, and use the money to buy brand new non rusty parts.

How much will you give me for my '70 Coupe shell? There are no doors, no fenders, no hood or trunk or gas tank, no interior, no dash or gages, no exhaust, no trim, no wiring, no steering parts, the rear quarters are bad(one due to being hit), the core support on the opposite side is tweaked, probably from the same accident, the windshield is cracked (there's no other glass there), and I need to patch the floor pans due possibly to some off-road mishap of the original owner. Oh, and the battery tray area needs replacement. Also, it's a '70 body with a '69 core support on it. I think someone made it into a mock '69 at one time. Please tell me how much that's worth and can I buy even a set of full length floors for that much?!

I can tell you that I have much less into the coupe than a set of full length floors cost - and that's with towing it across a couple states.

I'm taking the coupe shell to a frame shop tomorrow to make sure it's straight before I start working on it. You can still buy it tonight if you think it's worth enough to pay for all those new parts you want me to put into the convertible!

edit: all joking aside, it's very important to me that the end product is straight and will function well. I'm a mechanical engineer, but I don't think I have the ability or determination to build an entire '70 Mustang convertible pretty much from scratch and get everything as straight and true as I want it to be. The convertible I own is just too far gone - maybe a pro could fix it for $15k + parts or so. I doubt I could do a good enough job to satisfy myself at the end.

second edit: too bad this thread was a total waste. I thought with the number of people on this site who try to improve their cars I'd get more than just a couple purists who think the original design is perfect. You're welcome to your opinion, but I don't come into your restoration threads telling you you're wasting your time and that restorations are boring and drive like crap. Do I?