GM to Mopar trans swap?!

Yes. You can look at the transmission and identify it from appearance. You have a service manual? Latest I know of to download (and it's free) is a 73 which would be very similar

http://www.forabodiesonly.com/mopar/showthread.php?p=1970088617

More manuals and info here

http://www.mymopar.com/index.php?pid=31

Some more info

http://www.slantsix.org/articles/4-speeds/3speed-info-swaps/3-speed-swaps.htm