Looking at your description, the answer isn't the main bearings or the pump.

Here's why:
If the engine will develop 40psi at normal speed, the pump and bearings are okay. Now, it wouldn't hurt to replace the mains, but again, if the clearances are tight enough to maintain 40psi at say, 2500 rpm, there is nothing that will cause them to loosen up as you go higher. And if the pump will pump enough oil to develop 40psi back pressure (which is what your gauge reads) it won't pump less at higher rpm, assumoing it has oil to pump.

You can check your bearing clearances with Plastigage and get a ballpark number. I bet it's less than many on this board normally run.

So the question is, why won't the pump PUMP ENOUGH OIL at higher rpm? Two reasons stick out in my mind:

1. There's not enough oil to pump. This could be because, a. Sludge in the engine is slowing drainback to the sump, b. The pan is inadequately baffled and on acceleration the oil runs away from the intake, c. There isn't enough oil in the crankcase, d. The oil is foaming, e. The oil pickup tube has some sort of internal blockage that is causing the pump to cavitate.

2. The pressure relief valve on the pump gets stuck open and most of the pumped oil goes back into the sump. This would usually display other symptoms as well, and is a little more far-fetched.

Don't buy anything except Plastigage until you have this figured out.

One of the key pieces of information is that the engine was a two-barrel grocery-getter engine and it sat for periods of time, then you put a different carb on it and somewhat drove it. That means to me that you don't know how it was driven or maintained before you got it. Low mileage doesn't mean lack of sludge.

R.