2003 4.6 Oil Pressure Problem

I am writting this in hopes someone could shed some light on me with a problem I am having.
I installed a 2003 Mustang motor in my 97 T-Bird. Everyting was pretty straight forward on this swap. However, when the motor warms up I get a very noticeable valve rap and when in gear at idle the oil pressure light comes on. I am using 5w-20 as recommeneded and motor has 60,000 miles on it.
I bought a mechanical guage and hooked it up to the drain plug next to the oil filter.
600 rpm - no reading
1500 rpm - 7 psi
2000 rpm - 12 psi
2600 rpm - 26 psi
4000 rpm - 37psi
Redline - 50psi
These readings were takin in Neutral. I am thinking that I have a bad pump. The guy who sold the motor to me thought the throwout bearing was the culprit to this valvetrain noise. I am thinking there was more than one problem now.
-Mike-

When I made the conversion I had to use the T-bird oil pan. I did inspect the bottom-end and top-end of the Mustang motor. From what I remember this motor was very clean with no visible varnish or sludge. After I noticed there was a valve rap, I did run engine flush through the motor. Did not do a compression test because to me the motor runs great, smooth idol, plenty of power, no oil burning or leaks. I did start her up today and the oil pressure went to 50psi at fast idol then dropped to 15psi. It almost seems like there is a defect or problem with the oil pump to where it works but only at 60% of what should be normal.
When the motor gets warms you can hear the cam followers tapping on both heads. A friend of mine said these need a minimum amount of pressure to keep them quiet. My first guess was a valvetrain problem but with low oil pressure I am thinking not.
If there was an oil blockage I would think the oil pressure would be high?

Thanks for you input and help guys.

I used the pickup out of the T-bird motor. It is different because of the oil pan configuration. I am hoping that I can lift the motor enough to slide the pan out to inspect and service the pick-up and pump. Pulling the motor back out would be a lot of work.