Your TH350c should be fine behind just about any smallblock. As mentioned before, you have a BOP pattern so it won't bolt directly to a SBC. If you don't abuse them both the transmission and rearend will hold up fine. The overdrive upgrade is a good suggestion but will likely need regearing in the rearend to be of any benefit. Obviously the SBC is the cheapest option and has the most aftermarket support. Since you already have a Buick in there it will probably be the easiest since the peripherals like fuel lines, battery cables, wiring, etc are already in the correct locations. I don't know about whether the motor mounts are in the right locations but that info is on the forum somewhere because I know the Buick V8 swap has been done a few times.
