The "square bady" chevy trucks that are rated 3/4 ton are much tougher than the 6 lug 3/4 ton made in 88 or the big boys made in 92 up. The axles are real truck parts. The unit is heavier, but parts are easy to get, the axles and things last forever. The torsion bar suspension has been tried before and was not as strong. 50-62? maybe?
I really like the 67-72 model, but it is not as plush. Changing heads might be fine for the SBC, but if you buy a BB with TBI 91 -96?, be careful changing heads. Those had these peanut port heads, if you use other heads, you could be in for coolant leaks.
There is a way to use a more powerful fuel pump and larger rails maybe injectors, but you have to flash an ephrom and it might be better. The peanut heads with the 454 pull great, but it is very flat and just like a freight train. Do NOT try to make it a race car, that it will never be.