Back in about 1972 I took the original 3 speed/Overdrive out of a 1957 3100 with a fresh 283 and put in a Muncie four gear from a 1969 Camaro. It bolted right up the the V8 bell housing.
FWIW: I was never happy with the combo and soon replaced that M20 or M21 with an SM 420.
I remember now that it was a dog when starting off, especially on a hill, the first gear ratio not being a good match, even though the stock O.D rear axle was 4.11. Can't remember now what size tires I was running on the 15x 7 or 8 "chrome reverse" wheels.
Rick O is providing some good advice here. I too have a M-21 Muncie in my '65 C10 (
http://talk.classicparts.com/showthread.php?t=13928 ). You'll need at least a 3.73 rear gear, and if you are running oversize diameter tires the 3.73 gears really aren't enough. But anything taller than a 3.73 and you'll lose any chance of highway driving due to increased engine rpm (
http://www.csgnetwork.com/multirpmcalc.html ).
One thing that would help the launch ratio (
http://www.5speedtransmissions.com/startlineratio.html ) is a heavier flywheel, especially if your 350 SBC is healthy and has decent HP.
I like rowing through the gears of a 4-speed too, but I believe you would be much happier with a 5 or 6 speed that has a steeper first gear and an overdrive final ratio.
