Dallas, if you saw a short "midshaft" to a carrier and then on down to the rear end in an enclosed shaft, that is a 1/2 ton GMC longbed. The short shaft made up the difference in wheelbase.

Right church, wrong pew.

You need a 1/2 ton 4 speed complete setup for your truck to convert it. Try Jeff at Bowtie Bits for help as they sell rebuilt SM420s.

http://www.oldgmctrucks.com