If you just rebuilt the motor, I'd bet your sending unit or gauge might be faulty. Try getting a reading with a gauge other than the stock one.
If it really is low, try shimming the bypass spring with a washer or two. (Extra oil pressure to the motor, not the cooler is a good idea. Your cooler will see plenty of oil when your running down the road, and it actually will have enough air flow to cool it before dumping it back into the pan.)
Just my .02