oil pressure and crankcase pressure

Doesn't the oil pump itself have the pressure relief valve built-in?? I would probably drop the pan and remove/disassemble the pump the check it out. You didn't mention the pressure you saw (got a gauge on it?) but I'm sure it'd peg over 100 psi if it blew the filter off the motor anyway, huh?

Any chance that the oil pump got any moisture in it before installing it on the 318? Maybe a small amount of rust on that valve is sticking it closed.

I don't think any restriction anywhere in the motor's oiling system is responsible.