I'm well aware of the purpose of the PSRU, I'd like to know if it's
feasible to convert an automobile (or other) engine to run at an RPM
low enough so that a PSRU wouldn't be necessary. I'm thinking a custom
camshaft would be needed, and different ignition timing, what else?

You need much longer stroke, which would mean a crank with longer
throws which won't fit in the crankcase anymore, and a longer cylinder
to accomodate the longer piston travel, which you're not going to get,
either. Take a look inside any auto engine crankcase sometime, and see
how close all that stuff is running; there's not much extra room. Chevy
took the 283 and made the subsequent 305/307/327/350/400 engines out of
it by boring larger diameter cylinders (which required just a bit more
casting thickness) and a crank with a tiny bit more throw, which they
managed to squeeze into the case. See
http://www.aces.edu/~gparmer/sbc.html
The Chev 350 V8 has a bore of 4" and a stroke of 3.48, while my
old Gypsy Major, a four-banger that had about the same displacement,
had a 4 5/8" bore and 6 1/2" stroke. Big torque. Redlined about 2650
rpm. Modern aircraft engines like the O-320 are more oversquare like
the auto engines, but still have longer strokes of about 4".
The car engine's crank, as someone else pointed out, won't take
prop thrust loads well, and certainly can't handle the gyroscopic loads
the prop places on it. Even the direct-drive conversions usually have
some sort of extension and bearings to take those loads; those that
don't, like many of the VW and Subaru conversions, have had crankshafts
break in flight. Special forged cranks are required, but the bearings
in the case are still too light.
Try picking up a Lycoming 0-320 sometime: 280 lbs or so. Then
try picking up the Chev 327, almost the same displacement, and see what
it weighs: 575 lbs. Then pile on the radiator and some water, too. Your
airplane has to lift all that.

Dan