A well know tranny builder in the Okanagan told me an M20 with the larger 1" output shaft was the ideal tranny for rear end gearing between 3:36 to 3:73. Anything beyond that M21
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"you can't burn out, if your not on fire" -Jim Morrison
Some people automatically assume the close ratio M21's are somehow better. They work fine with 4.11's or 4.56's but as Carl mentioned, an M21 is not a good transmission for a car with 3.36 gears. The high first gear coupled with a high rear end gear would make the car very sluggish off the line.