I'm trying to figure out what to do as far as a replacement transmission. Currently have a ZF 10M (on a Perkins M30 in a Catalina 320). My mechanic is urging a replacement (rather than rebuilt the existing). Two options - same ZF 10M or new ZF 12M. ZF 12 is touted as a replacement, but is 1/2" longer, the prop shaft is .4" lower, but doesn't extend as low. Anybody do this? Words of wisdom?