You know what? A 3/4 ton is noticeably bigger than a 1/2 ton, at least to my eyes, and while a smallish S-10 frame might work underneath a half ton, I would think for a 3/4 ton you'd want a larger truck frame.
I agree, I would not want to repair that frame either if it were mine. Someone else could have at it if they wanted to. I would probably be looking for another frame too.
Having converted to a small block V8 on the original 3/4 frame myself, I can honestly say it wasn't a real fun conversion. Everything needs engineering from scratch to make work. It is possible, but you have to really have the abilities to jump in there and start modifying.
And the one thing that comes to mind is that while the engine bay is very tall, it is also very narrow. There isn't a lot of side room at all.
So the first thing I'd be looking for is maybe using a Dodge Cummins Turbo Diesel truck for the re-body. The reason is because that's a straight six engine too, and I think it might work in your available engine bay. Plus that's a buttload of power those motors have. I'd start out by taking measurements of the frame, width, length, and wheel base. Minor changes could be made as you go. If you get a Dodge Ram with a really beat up body, but mechanically sound, this could be an interesting swap. Just plan on doing a lot of sound insulation of your cab!
If not that, and you really have your heart set on a V8, then I would suggest a Chevy C-10 from the 1970's. And do the same thing. Find a good fabricator willing to do the heavy work for you and welding the custom body mounts, and then go shopping for a donor truck.
Keep in mind surface rust is no big deal if you can sand or sandblast it off and repaint it. That steel is so thick that surface rust would take a long time to grow enough to compromise the frame.
