if you must use manifolds, how about a set off a 93-97 Z28/Trans AM/Caprice/Impala SS with an LT1? they are sturdy cast iron, and are supposed to flow pretty well. kinda funky looking, but they will give you a perfect port match, and supported over 300 horses from the factory in the ram air Firebirds. ask around at the local exhaust shops to see if they know of anyone that put headers on their LT1 powered car, or look on ebay or in the for sale section of this site.
the sad part is that i used to know of a set from a 95 Z28, but they got thrown away last summer.
you don't plan sincerity.
you have to make it up on the spot.
wanna hear about 20 years ago when i was too smart to know any better?