Check to be sure there is no water in the oil. That sounds like too much pressure to be simply air unless the clamp wasn’t tight. It could be a leaking head gasket pressurizing the cooling system. Leave the radiator cap off, refill, but not too full, restart it and see if it will purge the air and watch for normal coolant flow in the radiator. Are you using break in oil and additives? Also, you should alter the rpm during break in, not steady rpm. Occasionally slightly revving the engine will help assure that the lifters spin and properly bleed, etc. If this happened to me, I would stop everything and pressure test the cooling system to be sure that there isn’t a leak. If a head gasket didn’t seal, you could do major damage, especially during initial break in due to contaminated oil, overheating, or both. Can you rent, borrow, or build a pressure tester? If you are confident there’s no cross contamination going on, let it warm up and purge the cooling system with the radiator cap off, let the thermostat cycle and fill it as needed. It should not be full, you must leave room for the coolant to expand when it heats up, full hot will be several inches below the filler neck cool, if that makes sense. On a side note, are you sure that the water pump is rotating the correct direction? I had a nightmare while breaking in a sbc several years back. That particular engine required a reversed rotation pump, the parts store sold me the wrong one and I didn’t catch it untill it overheated during break in... not good. Just a thought.
