I had a customer years ago that went through a similar ordeal with a 340. He finally brought the car to me. Knowing that he had changed the rear main seal, pan gasket and oil pan several times and pan gasket several times, the first question I asked was,
who assembled the motor? He said he did. Next question, Are you sure the galley plugs are sealed and tight? He said yes I distinctly remember doing them myself.
OK, so we proceeded to replace the rear main and pan gasket 1 more time. In the process I reinstalled his original oil pan.
Primed it, fired it up and within a short time the leak appeared.
Fortunately it was a 4 spd and we were able to drop the inspection cover on the bottom of the bellhousing.
I proceeded to shoot two full cans of brake clean up onto the the back of the motor. After several minutes I poked around pretty thoroughly with a wadded up paper towel. It was desert dry.
We started the motor again and shut it off as soon al the leak appeared.
using the same paper towel I went up the back of the motor staying away from the rear main area. The towel came back with fresh oil.
I commented, Oil does not run uphill.
We removed the trans flywheel etc with him watching. The RR galley plug was only finger tight.
To my knowledge the car has never leaked a drop of oil and that was almost 18 years ago.
I mounted one of his new pan gaskets to a board and put "Think outside the Pan John " on it.

Not sure what you have going on but, if you have that much crankcase pressure to blow out the rea main seal with open breathers you have other problems like ring seal. And breaking it in further is not going to resolve it.
One can attach a fuel pump tester / vacuuum gauge to the dipstick tube, and seal the breathers. Start the motor and see how fast the gauge builds pressure. If it pegs immediately you have a ring problem. If not start looking elsewhere. Any chance of sneaking a inspection camera up in the back of the motor area ?