Is the heavy oil required for your engine? e.g.--loose clearances, a pan that isn't really up to the job? Sometimes it's a necessity but usually only in circumstances where a stock pan must be used and the engine spins high RPM for a good while (where oil temp will get up there and stay there). Usually a dry sump would cure those problems but anyway...
What kind of clearance do you have on the mains & rods? Cam bearings oil clearance? How far is the pickup from the bottom of the pan? And is there a gasket on the pickup tube?
50 wt oil is going to take longer to build pressure because it's thicker. The pump has to SUCK the oil from the pan before it can move it through the passages. Then once it hits restriction, it will build pressure. If there is any leak between the pump & pickup, it'll take forever and a day for the pump to prime. Once it does, it has a tendancy to aearate a little more which is not the greatest situation for the bearings. Then since it's thick, it's not going to want to flow to the restrictions quite as well, so it'll take longer than normal to build pressure. I have always had the best luck with synthetic 20w50. Year round. Oil temps stay down and pressure is rock steady at 55 psi no matter what RPM (from 1400 idle to 7800)
Where are you taking pressure from? Front of block by the filter or?
Have also seen pressure relief valves STUCK in the Melling pumps. And a LOT more often lately! Dad's stuck SHUT and exploded the oil filter (and gauge), then oiled the tires and he went over the wall. Destroyed the car but engine & trans were ok. I had one stuck open on my meth 438" and it would build pressure slowly and then it stayed lower than normal (40-45 psi). Pulled the pan/pump and fount a stuck relief. Replaced the pump with one I had already "worked on" and all has been well.
50-60 psi is plenty with these engines unless you're running Nitro, nitrous, or boosting the heck out of it with loose clearances and in those cases, you'd better be changing bearings often anyway.