Is the engine out of the truck on a stand or still installed? Fresh rebuild or well worn?
How much oil pressure is the engine making?
Solid or hdraulic lifters?
Is the bypass oil filter plumbed correctly,?
If not it can rob most of the engines oil pressure. You might try plugging it off and testing. Don't bypass it, plug it off at where pressurized oil leaves the block and feeds to it (usually where the oil pressure gauge line connects).
Reason I ask is that I recently rebuilt a 54 235 and as you have determined the oil to the valve train goes through the rear cam bearing. On a new rebuild one needs to be careful when installing the rear cam bearing. It can be pressed in too far blocking oil passages or the bearing hole not aligned in the block. The rear cam "freeze plug can also be driven in to far which could also block oil passages.
If your engine is well worn in the above cam bearing/freeze plug suggestions won't apply and I'd then suspect crud in the oil pump feed to the block or passages in the block.
Good luck
Dave
