Thanks for all the advice. I was looking on Ebay and there are several muncie trans. Which one would best to use. I have a 10 bolt would like to upgrade to 12 bolt with 3:73. 350 engine, possible upgrade at a latter date.

I run 3.73 gears, and an M-20...nice combo, but that is about as steep as you want to go with the higher first gear (2.52:1) in a Muncie...

Really, any of the three transmisison would work with w 3.73 gearset.

I specifically looked for a Muncie with the speedo cable on the driver side, because I was not concerned with originality, and I then would not have to change the speedo cable...

The factory breakpoint between the M20 and M21/M22 was 3.55-3.73. Factory specs called for the 3.55 for the M20 and the 3.73 as the start for the M21. Though the M20 will work with the 3.73, you won't be able to "grow your rear end" very well.

I went through all of the calculations about 9 months ago when I was trying to determine the gearing for my converted Turbo 350-to-M20 '68 Camaro. I opted for a 3.55 since that would also work well if I converted to an auto w/overdrive (which was the plan at the time). The 3.55 works well with the M20... or what I thought was an M20. My wife and father-in-law kept saying that it didn't feel like a "wide ratio - more like a close ratio". (Yes - my wife can tell the difference. It is her car, by the way...)

After we decided to keep the Muncie, I pulled it to inspect and change the clutch. While the tranny was out, I decided to check the case numbers - turns out it's a 1970 M21. I verified by manually checking the gears. Yep - the guy we bought the car from thought he had purchased and installed an M20, but instead had a M21. (Another note - when we bought the car from him, he had not changed the rear end from the original 2.73 gears that came with the car when it was an automatic. That M21/2.73 combo provided some interesting starts...)

The reason I mention this is that our 3.55 works ok with our M21, but had I known that I had an M21, I would have gone with a 3.73 rear end. But the good news is that if I decide to bump up the rear more - say to a 4.11 - I've got the tranny to handle it. And that's the point... If you want to run the 3.73 with "room to grow", go with the M21 or M22. But as Keith said, the M20 will work with the 3.73 also, but I wouldn't go any larger.