Time to rebuild?

All cylinders 130-140 psi sounds like "as new" compression to me. Has anyone ever measured higher in a slant w/ standard camshaft? Hard to square that with the leak-down hissing, so I would question the test method. Is the mechanic certain that each cylinder was at the correct crank angle when tested? As a sanity test, turn the engine over by hand. One can usually do so on an early slant by pulling and pushing on the fan belt, though your 1974 may have a shroud in the way. If you feel 3 strong "air springs" fight you per revolution, and you hear each one hissing down slowly (>5 sec), I would say your compression is excellent. Anyone who has kick-started a motorcycle would understand.

I can't imagine gasoline getting to the air cleaner by blowing by the rings. I would suspect your carburetor is sometimes over-flowing or spitting gas, which would explain it getting into the air cleaner and also flooding some cylinders to get in the oil. Most likely cause is a bad float needle or a fuel pump with too much pressure. Sometimes people drop the needle out of the carb inlet fitting without noticing (me, Carter 2 BBD, smarter if I stopped working when it gets dark). What carb do you have? You could try an old in-line pressure-reducing regulator (dial wheel, Mr. Gasket and other brands) if high supply pressure is the problem.