Be aware the 1/2 ton rearend is not a bolt in, the driveshaft length on the 1/2 is fixed. The shock mounts are different there are some modifications that will have to be made. If you elect to go this route be advised a 1/2 ton frame and a 3/4 ton frame are very different. You may be better off finding a correct width rearend from a more modern 3/4 ton (it has been discussed here before). Then matching that to the new trnasmission/driveline you select. If you elect to keep with the SM-420 you will need to find a 54-59 open drive version.
