Is this beyond hope?

[Shayne]

What is the tranny called? I have a 420 and it isn't the same as that.

[Grant Bowker]

I don't know of the CMP Chev transmission having a name, it was simply the 4 speed truck transmission.
For a while I was looking at trying to find a SM420 to put into my C15A. In the end I decided it wasn't worth the effort. Although the SM420 is compact by today's standards it is still quite a significant bit larger than the CMP transmission. Putting one in would mean either shortening the already short driveshaft between transmission and transfer case or moving the transfer case and as a result the mounting crossmember and shortening the rear driveshaft. Life would be simpler for someone with a 2 wheel drive CMP, only one driveshaft to shorten.