Low oil pressure from 351M

I've had a few of these 351M motors in small mason dumps and pickups for work. A couple traits they all shared was they made very little power (2 bbl motors) and all had terrible oil pressure, a couple reading zero at idle on the factory gauge. I always tried a new oil pressure sending unit first which sometimes resulted in a little higher pressure on the gauge but in my experience never an acceptable reading. Second, an oil change and filter with thicker oil always seemed help a little. A ford mechanic once told me they had narrow main bearings which wore causing the low oil pressure readings. In their defense I never had one blow up and we ran em' loaded down pretty hard.