7164 is a oval port manifold- and you stated you have rectangle port heads, a bit of a mismatch there??? Pics and casting #'s would help.
I prefer single planes on jet boat motors. But have seen many dual planes on 'em too...
I had a eddy carb on mine for the weekend, while the holley was being rebuilt, and lost ~200 rpm.
But then again a Vacuum secondary Holley 3310 shouldn't work on a jet boat, either...IDK???
What impeller do you have now, and what RPM will it pull?
Dan'l