Steve.. What an unexpected twist!!!!
I am racking my brains as all possible reasons have been explored. Just see if the problem is cured with the rocker feed disconnected, oil fed to a catch can. This feed circuit is always adding to oil in the sump. I suspect the design capacity of the scavenge is on a bit of a knife edge, too much oil here may be just tipping the balance. Might be as simple as a blocked pipe in the tank, forcing the return oil to feed back to the sump via the rockers. Keep an eye on the tank level, don't run it dry.
If no improvement, the scavenge side is the easiest to investigate, so this starts with the sump ball valve and ends with the tank. You are looking for obstructions, and leaks which will allow the pump to draw air instead of oil. I would have the pump off and make sure the scavenge pick up is sealed to the crankcase as described in earlier posts. Whip the pump back plate off, just to see if all is well, scavenge gears correct, no endfloat in the chamber, no leaks on the pump, pump /crankcase gasket sealing OK, and not blocking or obstructing any oilways. Are the holes in the new pump in precisely the right place to accurately match your crankcase?
If you consider there is more oil being delivered than the scavenge can return, this would indicate an oil leak in the delivery side. See if it is possible to blow easily down the oilway to the anti wet sump valve. Probably need a low pressure airline, to lift the ball but there should be a fair resistance to air flow. No back pressure means a leak somewhere, which you will hear. If this seems OK, worth trying it with the old pump.
Swarfy.