If that is the case block or heads have been milled. I would probubly start with some shims and figure out what thickness fits, call Cometic Gaskets and have them make you some. Another option is mill the bottom of the manifold. Years ago I was running a chev on Nitro in a Dragster. We had milled the block to get the deck height we wanted and angle milled the head to get the chamber size down, then milled the intake side of the head to get the correct angle to the intake side. Proper fit would require milling the intake, a coveted injector manifold. I refused to mill it so I had to hand make intake gaskets from .020" material.