Aftermarket manifolds in general on street engines don't impress me much. Anytime an adapter has to be used, it diminishes the efficiency of the manifold. Going from a smaller carburetor to a bigger manifold has less of a detrimental effect than the other way around, but neither approach is as good as an original equipment manifold and carburetor. The small bore Rochester 2 barrel is probably the best of a bunch of bad choices, if you insist on using the aftermarket intake. I'd start looking for a good condition rebuildable Rochester 2 BBL for a 1955-57 265 V8 as the best option for a 2 barrel on a 250 inline six, or one for a Mercruiser inline 250 equipped with the Rochester.
Jerry
