The configuration of the intake manifold is as important as the choice of carburetors; as the air/fuel mixture MUST pass through the intake to get to the cylinders.
No actual test data to back me up, but I would (opinion) certainly agree with Panic about the bias to the end cylinders being superior to bias to the center.
There is test data to support this on dual quads on Pontiac V-8's.
If doing a homemade intake, remember that the air/fuel mixture delights in traveling in a straight line and hates going around corners. Also that runners to each cylinder should be as close as possible to the same length.
As to a 2->1 adapter; the results of testing we have done (and we have done quite a bit on this subject) have ALWAYS been uniform.......regardless of what 2 barrel may be used, it NEVER worked as well as the single barrel it replaced. Think NASCAR restrictor plate.
Jon.
