CAM AND MAIN BEARINGS SHOT

Could be clearance, lack of oil.
You said there was no dry start up, that you primed it...so..

One problem 340's have is the main bearings oil holes do not match the blocks oiling holes. I open them up to the blocks passage size which is bigger and over to the pass side a hair. Another one is losing a lifter from a bent or broken push rod...that happens and th oil psi dips immediately and the rest suffers.
I see you have the full groove front cam bearing that allows more oil out the cam thrust plate. Are those full groove mains too? I run the same, still needs corrected at oil passages to line up to not starve it higher rpm. The oil goes through the pass side lifter galley and down to those mains..then at the no.1 main it feeds over to the drivers side lifter bank. I think running the no.1 main tighter than the rest is a good idea to keep the psi loss minimized. Not tight per say, just tighter than the rest. I'd run .0022- .0024 and the no.1 at .0019 for instance. I set my stroker
Up that way and it had the best psi of all of them so far, even after I shattered a piston and cylinder wall at barona speedway.. the bearings looked perfect...never starved of oil and I didnt even prime it at start up.
Look close at everything...check all plugs, line up the main bearings.
That's my wisdom.