My initial reaction was to mention the clearance between the oil pump pickup and the oil pan floor. It could be just a wee bit to close and restrict the oil flow reaching the pickup. Folks have reported here that they had a similar situation as you describe, higher rpm's and the psi gauge was going lower. And like Roger says, just replace the pump. I have zero idea why anyone would re-use an oil pump on a fresh motor rebuild. But if you do replace the pump, measure the pickup distance from the block - then measure the distance from the pan flange down to the floor of the pan. Make note also if the gaskets are being included in your measurements. Once you install the new pump with pickup. Repeat those measurements and verify you do NOT get the pickup screen any closer to the pan floor. I'd ideally try to increase that gap - even a little bit - just to remove that as potential problem. Glenn is our resident SBC expert. let's see what he offers as both Mr. Spears and myself bleed Ford Blue! LOL