I have a 1988 firebird with a 305 H.O. tbi. Id like to turn a 350 that i have into a 383 with about 350-400 hp an put it in my firebird. Any thoughts on this? go bigger, too much, frame troubles, rear, susp.?

Every one of our cars should have subframe connectors. If you don't, you really need them with that much power.

Plan at least on beefing up the rear end. You may have 26-spline axles stock, 28-spline are much stronger. Requires a different carrier, good time for an aftermarket posi carrier. Aftermarket gears, welded tubes, aluminum support cover will all help it live behind more power.