To answer all of your questions at once, dad changed the extractor on all of the M1s that he worked on so that if the port pressure become to high it would quickly show some damage to the extractor groove on the head of the case. Start with a light load, and increase it a bit at a time until the extractor marks the groove on the case head and the next charge back from that is the do not exceed. At least they M1 is not as sensitive to port pressure as the Remington 742 and 7400 rifles are. I have seen loads that worked really well in M1 Garands completely wreck those rifles, because the gas port was closer to the chamber.
Best wishes,
Joe