The P type has a larger one-way clutch and more surface area on both sides of the cone clutch. There have been thousands of high-miler J types behind turbos. All 240 turbos were j types.

The trans output shaft is different for the P type so it is not a bolt on replacement for a J type.

The M47 is made from the same 1st through 4th parts as the M46. The first gear brake is the most troublesome part of an M47 but that can affect a late M46 as well but there were few late M46's so it seems to be less of a problem (by "late" I mean 86 or newer). Most all 5 speeds are inherently weaker due to the nature of how the 5th gear is added on. To my knowledge the 5th gear portion of the M47's has never been troublesome. The biggest killer of M47's is use of ATF. In Europe they use synthetic oil in them and have few problems. They came from the factory with synthetic oil.