I'm running an Offy intake with a 292 exhaust manifold. I got the combination to bolt together by doing some careful grinding on both pieces. On the intake I had to grind away at the coolant passage but since the 292 exhaust bolts directly to the intake there won't be any coolant in the passage.
Once they bolted together I took them to my engine builder and he machined the two gasket surfaces smooth. There was about 1/16" difference in the two surfaces.
Added benefit: a single 2.5" exhaust is easier to DIY than a split manifold. For my intended use the 2.5" exhaust is more than adequate.
