you're much better off, if you're really looking for more power, to build a 350 or 400...
one piece of advice though... i had a TPI system in my 1985 350 corvette, and i'll warn you right now, unless you make some MAJOR breathing mods to the intake it isn't going to make much top end power...
the TPI was designed to boost torque on a smaller 305 engine. when you bolt a TPI setup onto a 350 or larger engine, it runs out of breath REAL fast! i will admit it'll make some brutal low end torque with the right cam, but don't expect any high rpm (5000+) power out of it...
there are a few aftermarket replacement manifolds on the market, TPIS and Edelbrock to name a few, so you might want to look into those...
good luck!