Oil pressure dropping

[Waterloo]

Why did you use a reman head? That could be your oil pressure loss right there. You should be hitting 40psi easily with that new pump. No shimming required to get to 40psi either, especially after a fresh in frame.

The sad reality is, they may have simply cleaned up your old head and reinstalled it.

You did check the serial numbers on the head, or were there to see it installed and your old head carted off to the scrap yard?

This is getting to be pretty common out here... Something to check, the head serial number should be welded into the back side of the head, at the firewall. I would check.