I have detailed the conversion (probably in bits and pieces in several posts) on the forum. Please use the advanced search function in the C1/C2 section, and search for "MY6", and all posts by "plasticman" (in both current and archieved threads) .
Roughly, the MY6 was produced by New Process (Chrysler) for GM during 80 through 85 or so, for the full size light pickups. It is based on the Chrysler A-833 trans, which has a good reputation for durability and strength. There were 2 different bolt patterns (which require a different bell housing for each). I think the 1st version used the standard Chevy bell housing (truck version with the larger center hole, note the pass. car version that our Vettes came with). The truck version bell housing is still available via E-bay, bone yards, etc. The 2nd deign had an odd bolt pattern (that solved an interferance in bolting it up), but must be used with it's correct bell housing that was probably not used elsewhere!
If you are planning on putting this in a Vette that has the clutch stud bracket bolted to the side of the bell housing, that is an issue since the truck bell housing do not have that provision. I made a separate bracket, and has worked just fine.
The rear trans mount must be modified and spaced to work but again a no-brainer (at least in the C1 Vettes).
The overall length and splines/shafts are identical to the Muncie. No driveshaft mods required at all.
I and wmf62 used a Hurst Comp+ shifter, but the rods must be altered, and at least one lengthened. The mount for the shifter also needs to be designed or altered, depending on which shifter you use. My shifter tunnel hole had already been "enlarged" by a PO, and my shifter fits right in the same location, but on a new mounting plate. WMF62's 62 uses the original shifter hole, and we made a new mounting plate to keep it in that position (his Comp+ Hurst is different from mine in several regards, but they both "work").
The speedo is another issue, but a speedo ratio adapter box (easily available for $100 or less) fixes that (once you know the correction factor needed for your application).
The MY6 weighs about 30 pounds more than a comparable Muncie (only a rough guess). It is deeper in the main case "belly" than a Muncie, but otherwise about the same dimensions, and no tunnel mods required, and no issues hitting anything.
It has ratios of "about" 3.09 1st, 1.68 2nd, 1:1 3rd, and .73 OD 4th. Speed shifting is not it's strong suit. It is a very wide ratio box that needs an engine with a wide torque band. I am running a strong 350 with dual AFB's, and wmf62 is running a LT1 (350) with FI.
We are both very pleased with the results (and they have been in our Vettes for +1-1/2 years now). However, do not attempt to downshift this trans while rolling faster than 10 mph from 2nd into 1st gear (unless you double clutch, and 3rd to 2nd must be done with more care than a Muncie). Upshifts are no issue, but are not as fast as a good Muncie.