Tell that to the F.A.S.T guys who have more inches and run down in the low 11's through a stock exhaust system. I recommended he use the B Body HP manifold on the driver side and trim the bottom of the steering column for clearance.
I have used the 68 thru 70 Set of B body's especially the drivers side B body H.P. on many A body's with a big block. They work great. And a lot cheaper.