Jim,
If your 312 has the side draft tube (on the side of the block, just aft of the fuel pump location). Of course, I cannot remember if any 312's have the side draft tube, I assume that all of them had the rear vent off the valley pan, but I digress.
Anyway, if you have the side draft tube, something I have noticed that happens on occasion - the can does not seal properly like it should (you didn't say whether or not the oil is coming out of the actual tube or leaking down the side, so understand that this could be a shot in the dark) that old can has a tendency to be just a tick smaller in diameter than the diameter of the tube/bracketry attached to the block. The seal sits in there nice, but the can is very, very easy to get a little cocked without even realizing that it is. Could be your trouble, hope this will help.
Dan
Daniel JessupLancaster, California
aka "The Hot Rod Reverend" 
check out the 1955 Ford Fairlane build at www.hotrodreverend.com