Kevin,
As you have already discovered from the text in DIYFord (actually lifted out of Barry R's book - How to Build Max Performance Ford FE Engines), Melling offer 3 types of
oil pumps. I would suggest you find out if you have a HV or HP pump in your motor. Also, are you running a remote
oil filter? Maybe a daft question, but have you connected it the right way round? Where are you taking the
oil pressure from?
If you are running a flat tappet cam, I would be wary against using a lighter oil. Your 20W50 VR1 is the right weight of oil for that type of cam. I'm running a hydraulic roller cam and I use Joe Gibbs HR oil 15W50. My engine usually runs up to 80psi from cold but then settles to around 40psi at idle (1100rpm). It could be that your motor is still tight and needs running in more? However, not being able to turn it over with a breaker bar (depends how long your bar is?) doesn't sound right to me.