I've been building SB's for over 20 years with the Melling high volume pump with the high pressure spring with no problem. I have had oil pressure from 60 - 100 psi and run them over 200, 000 miles.
I used a TRW high volume pump ONCE. It lost pressure at 500 miles on the engine. TRW claimed that they would pay for damage if it was their fault. When we inspected the pump we could not budge the oil pressure relief valve with the longest Craftsman screwdriver at the time. "When we sent it back to TRW they cliamed we "cleaned" out the pump. Their official "lab" analysis was, "The failure was caused by tiny minute particles small enough to fit through the stock oil pickup screen" that got lodged in the pump and stuck the valve.
They rebuilt the pump and sent it back to me and would not cover the cost of any damages to the engine. (how nice of them to rebuild the pump for free for me - thanks guys). If their pump can't handle anything that the stock pickup tube screen can't filter out, it is junk to me... I could not trust TRW after that, and sold the "new" pump to someone else for $25 after telling them the story and told them if it blows your engine, you take it up with TRW - I want nothing else to do with that. I still to this day, do not buy any TRW parts because of this. I have since been using the Mellings/Mopar pumps and they have run fine with no problems.
OK, now for your pressure inquiry:
I would say 45 to 100 is good for an engine running above idle. I use the minimum 20 psi at idle rule. As long as you can get 20 psi or more at idle (in gear is better), you should be ok. I think the best way to explain it is as long as you can get the oil there, let the oil do its job. There is a phenomena called "boundary layer lubrication". This is where you have a journal and a bearing shell with oil between them with tight clearances. The oil will form a boundary layer and not let the two metal pieces contact each other. Due to the tight clearances and the layer of oil (which cannot be compressed) the oil will cause the two pieces to "float" and not let them contact each other. It's similiar to hydraulic lock. You can't compress a liquid. Then the oil can support the load between the journal and the bearing shell and keep them from contacting each other. This keeps the two parts from contacting each other and wearing.
To test this you can perform this simple test. Take a large piece of sheet metal (let's say 12" square). Then pour some oil on it right out of a new bottle of oil. Then take another smaller piece of metal (let's say 1 or 2 inch square) and place it on the oil. Use a stick or rod and then try to press the smaller metal piece to the larger piece. It can move freely from side to side and not ever contact the larger plate (making sure there are no sharp burrs on the edges).
So basically, as long as you can supply a decent amount of pressure (45 - 100) and keep the parts supplied with oil, the boundary layer lubrication will take over and prevent the metal to metal contact which wears the parts. Just get the oil there with enough volume and pressure and have a good oil. In a racing condition, I would be more comfortable with a pressure of 50 - 60 minimum over 6000 rpm.
