So is the M12 the same thing as a T56 but with different gears? Is there anything different about the shifter? I'm looking for a 6 speed for my 79 T/A conversion and found an M12 for $2,000 that I can pick up.

 

3rd and 4th gear have different ratios from what I understand. I'm not sure on the input shaft length from the GTO's though...

 

1,2,3,5,6 have different ratios.

MN6 T56 and M12 T56
1st 2.66 2.97
2nd 1.78 2.07
3rd 1.30 1.43
4th 1.00 1.00
5th 0.74 0.84
6th 0.50 0.56

Also the M12 has carbon blocker rings and triple cone syncros. All in all the M12 is a better unit, but make your rear end gearing choices accordingly.

 

arent all the new tranny rebuild kits carbon blocker rings now?

 

06' model summary

Stepped clutch teeth on synchronizer sleeves (M12)
These staggered teeth sequentially engage the gears, which makes engagement feel lighter and more positive to the driver. This modification originally appeared on the 2005 Corvette. -this revision is speaking of the CTS-V
High-density carbon blocker rings (M12) -the first carbon blocker rings were introduced in 02'
More durable carbon friction material is used on the synchronizers.
Stronger snap rings on synchronizer hubs (M12)
For the first-second gear hub, a stronger snap-ring is used to increase durability.
Carbon fiber reverse synchronizer (M12)
To reduce efforts to engage reverse, the synchronizer friction surface is changed to a carbon fiber material. (was brass) Reverse-inhibit mechanism to block the shifter from going into reverse when the driver is aiming toward fifth or sixth gear gate with the shifter is also modified to make it easier to engage reverse.
Revised shift precision (M12)
Tolerances for the shifter mechanism for the Cadillac CTS-V application have been tightened to make the shifter feel more precise.

I believe the M12 was introduced for the 01' Z06, it's a slight step up