[Mr. P-Body]

While crate motors are "easy", they are hardly what the hype cracks them up to be. I know of at least two of those 502 horse 502s that have needed mass amounts of rework to make them actually MAKE 502 horsepower and stay alive, at the same time. Like any Chevy engine, machine work is spotty, and the "blueprint" is WAY off. The first thing we do to a Chevy motor for racing, is to take it apart and make all the dimensions equal ("blueprint"). Craters are no better than production line stuff. They ARE production line stuff...
For a car as heavy as the T/A, if you MUST stay with a small block, get the Eagle 383, or have a 383 custom-built. 400SBs are too weak (block) for serious use, without expensive block work. 350s don't have the low-end capability the longer stroke has. We've had many happy customers with 383s. I would say it's the best of all possible worlds, with a street small block.
Putting a big block in there would certainly make it faster, but would totally destroy the balance of the car. The Pontiac is the compromise between the two. It weighs enough less than the BBC, to make the handling okay. It makes more torque, and ultimately more power, than the small block.
Sorry, no magic solution, but maybe you have a little more to think about, to make a logical decision.