I've got an RPM airgap on my 302 block, and while my current air filter setup doesn't work, it's only because I have a 3.5" tall filter element on it, and the bottom of the filter housing is an inch tall. Misses by about 1/2". I ordered a 2.5" tall filter element, will show up this week, and I'm confident it'll fit. I'm not sure what the nominal height difference is between the flange on an airgap mounted on a 302 vs an airgap mounted on a 351, but I bet it would be easy to find out :^).
All the best,
Chris
Big engines in little cars.
289 FIA, 347, Tremec 3550, Edelbrock performer RPM heads, air gap intake, hydraulic roller 227 intake 234 exhaust at .050. Ford Guardsman Blue paint.