OK, the pieces are falling into place thanks to you folks' help. Again
I apologize for giving less than complete info - should've realized all
this stuff is interrelated. If I've got the numbers decoded correctly,
the engine is a '74 Chev truck 350. It's in a '38 Plymouth coupe that
probably weighs under 3K lbs. Supposedly the engine has about 50K
miles. It doesn't burn oil, maintains a consistent 190 temp and, aside
from the aforementioned starting/idle problems (which don't seem that
serious), runs good. It's stock except for Mallory ignition,
Edelbrock manifold, Holley 570 cfm 4-bbl, and dual exhaust on OEM
ram-horn manifolds. It's hooked to a 350 auto with shift kit and a
Nova rear - haven't checked the ratio but I expect it's in the 3.00 -
3.50 range. The air cleaner is after-market - basically a 9-inch round
filter sandwiched between top and bottom plates. Carb freezing
shouldn't be problem as I don't drive the car in cold weather and
certainly not in snowstorms. For better or worse, we don't have
emissions testing around here.

Apparently the PCV grommet and the oil cap are OEM. From the
construction and fit of the plug, I assumed it is as well. But it
doesn't have a part number like the PCV grommet and the Chevy parts guy
couldn't find a listing for it. So now I'm thinking somebody blocked
the breather hole with an after-market plug when the OEM
manifold/carb/air cleaner were replaced. At this point the simplest
solution appears to be replacing the PCV grommet and oil cap with OEM
and replacing the plug with an after-market grommet and push-in
breather - all those parts should be available.

Thanks again for everyone's help in working this out.