There's probably a #2 to #3 adapter, but that will add some height. Re-drilling the manifold will be tough, I would begin by threading 5/16" aluminum rod to 5/16-18NC and bottoming them in the holes with epoxy, saw off flush, then drill & tap the new (closer) holes partially in the rod insert.
I don't see anything small enough to work well without mods, and still fit your flange pattern.
Some Zeniths may fit and have interchangeable venturis, but IDK much about them.
