Very rough and quick measurements, but my manifolds are roughly 3" long from manifold to the carb mounting face. And the stacks are about 1 1/4" long, from the front face of the carb. And there's about 2 1/2" of clearance from the front carb stack to the inner fender. Not quite as tight as I thought it was, but some of the taller air cleaners would be rubbing. And longer manifolds would eat up that 2.5" pretty quickly.

PS: Another clearance issue arises if you have B16 style hood hinges, vs. the later B18 style. The B16 hinges swing back pretty far when the hood is closed, on my car (which is a '63 and should have the B18 nose, but was probably crashed/repaired decades ago) they nestle down wight next to the fuel inlet on the front carb. Actually, first time I installed them, I had them sort of rotated over to hug the top of the carbs a little closer, and the hinge pinched and cut the hose when I closed it the first time. Started it up, fuel went all over the place, very entertaining. Took me a little bit to figure out why the fuel line got cut.
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)