Jim,
I believe you said your engine was a 64, that would make it a 422 casting. There was a change in the block for 64. Does your block have this hole?
Do you have a vented oil filler cap or sealed?
This is the main change to the block design and adds a big vent to equalize the pressure between the crankcase and the lifter valley. I believe this change occured with the beginning of 64 production.
The draft tube was replaced with a positive vent system that reversed the flow and now took fresh air for the crankcase from the air cleaner into the back of the block where the road draft tube was. The oil fill breather cap was replaced with a sealed cap and PCV valve attached to the oil fill tube and hose to a manifold vacuum source at the base of the carb. The big hole in the front china wall should eliminate most of the drainback problems caused by the drain slots trying to serve simultaneously as both oil drains in one direction and vents in the other direction.
Prior to this change, the oil filler tube cap served as the intake breather directly to the crankcase on the front side of the china wall behind the timing gear and then the air would have to work it's way up through the cam area drain slots in the lifter valley floor against the drainback flow of oil. The lifter valley was then vented through the oil splash shield on the bottom of the intake and then out the road draft tube. The road draft tube had an angle cut on the bottom to use the outside air while in motion to help scavenge the air/vapors from the engine. The problem with this is that if the crankcase is over pressurized or insufficiently vented, that pressure coming up around the cam, through the lifter valley drain slots over the cam hinders the oil in the valley from draining back into the pan and the pressure can build up in the valley preventing the oil on top of the heads from draining into the valley.
I see the problem, or at least part of the problem likely to be in your venting system as Jim_SS409, Ronnie and you have already mentioned.
The original oil pump moved 4.3 gals per minute @ 2000 rpm...doesn't take long to empty a pan a that rate.
JMHO