How fast have you gone with exhaust manifolds?

is expensive but if limited to manifolds, it can be worth it. The problem is that Chevy built a lot of manifolds with increased id and once they have been enlarged by te exrude process it's hard to overcome. I had a 56 Chevy and was lucky enough to come across a set of the maniflolds and in the late 60's I was offered a new set of headers for them. That alone tells you how good they were and what the Chebby croud is willing to do get their hands on them.