Right now I am running everything stock on my '91 z-28, I plan on starting with getting new runners and port matching the plennum to the runner dia. I assume i would need to port the stock manifold as well to see any significant gains in HP am i wrong? I am fairly confident with my porting skills. What do you guys think?

 

From my experience, you will not realize or notice much gain to the intake side without first doing modifications to the exhaust side...
Headers, and/or cat-back exhaust system and SubFrameConnectors are about the best 1st modifications you can do. Intake mods and a little port matching will not hurt you, though. I had no performance improvements from intake mods (other than cold air induction) until after headers.

 

The more work you do to the stock base the better. Go for it all, enlarge the entries and match to runner diameter, and match to heads. With bigger runners, and the ported upper plenum you'll notice a difference w/ or w/o exhaust mods.