Hello, I've been a reader for a while, but this is my first post. I am converting my 72 Skylark from a Th350 to a muncie 4 speed. I can't decide between and M20 or M21. I originally wanted the fine spline 71-74 M21, but later learned it would not be a direct fit with the Th350 driveshaft length, only Th400. Now that I'm back to square one, I can't decide if I want the 2.20 or 2.52 gearing. I do Street/Highway driving, and don't have any track plans in the near future. I have work done to the motor.. probably has 350-400hp on a Buick Small Block 350. So my questions are -Do I go with the M20 or M21? and which years in specific for a 72 Skylark? -Depending on the M20 or M21, what should I run in the differential if I go posi in the future? -Also what do I need to know about the reverse lockout situation specific to the 70-72 Larks? -Will I need a new speedometer or cables? I went through the stickies already, and apologize if I missed anything.