Oil pressure dropping after rebuild

[aruss99]

Hey guys, hoping I can get some help here as this is my first engine rebuild and I'm kinda stuck on what to do.
I have a 1971 C-10 that I put a 5.3 LM7 with a turbo in my truck and I recently rebuilt the motor. It has all new bearings, new cam, valve springs, ls9 head gaskets, and rod bolts.
Ive already rebuilt this motor once before and I'm having the same exact issue as I had before so clearly what I'm doing isn't working.
When I first started the motor, I had good oil pressure (not sure what is was exactly because my gauge in the truck just says L to H) but it was most of the way up. But as I let the motor idle and drove it a little bit, oil pressure was steadily dropping at idle as well as at rpm. It has gotten so low now that I can hear the lifters clattering so I know something is definitely wrong and I will be pull the motor to investigate.
What should I be looking for? I am pretty sure I installed the pickup tube correctly but have no idea if the barbell is in the rear of the motor. I never took it out but I have no idea if it's something that could fall out or if it is something that needs to be removed to put the cam bearings in.
Any help would be appreciated because I don't know what to do next since I thought I did everything correctly.

Thanks.