John,
A built TH-350 behind a 400-hp 406 Pontiac should be fine- 450 ft.lbs. is alot of torque but not enough to grenade a strong TH-350 (at least not for a while
). I don't think I want to run more than 600-650 hp, since that seems to be enough power to run 200 mph, as well as the limit of the TKO 600. Running any more than that is useless if you cant get the power to the ground, or burn it up. I thought 18 x 12's w/ 335/30R18's were overkill, but if you think about it that size rubber should hook up well (with the right tire).
If you jump over to the Pontiac forums there are plenty of Pontiac builders who have built 600-650 hp pump gas 462/468s, alot more hp with 541 IA2- based motors. The majority of those use CNC-ported e-heads, solid roller cam, forged internals, single plane manifold and a big carb, but that's a 600 hp motor for about $10k- not bad if you compare that hp/price to BBC packages available.
Geno