Stock exhaust replacement
 

I have a 79 Z28, 350ci, a little bit built up but not to much above stock. It has the original exhaust manifolds on it, the y pipe to the single catalytic converter, that back to 2 pipes on the 2 wimpy mufflers. I don't know the exact pipe size, but its small.

I'm looking into replacing the manifolds with long tube headers, cutting the cat off, turning it to true duals with flow masters, and putting 2 to 2 1/2 inch pipe on it. Would this make a noticable difference performance wize, or is it just going to free up some space and sound better?

Definetely make a difference. With the open exhaust it will create more flow but you have to make sure its not to much or you won't have enough back pressure and you'll lose power.