If you're already planning to swap the engine and/or trans then you could drop in a Pontiac engine and have more displacement and torque right off the bat probably.

That said, you may need to modify the engine mounts. If you swap the trans that would probably mean new/modified drive shaft and trans mounts. 700r4 used a TV cable for trans control which can be a pain, opposed to a simple kick-down for TH-350/400. Being that you already have the SBC most everything else should be good I think.