[David R Leifheit]

The only good way to change the rear is the entire assembly. Finding a 1/2ton with 6 lugs off a 60-62 is your best hope. Anything else will require the spring perches being changed. I -had- many years ago a 3:08 which came out of a heavy car and had the spring perch altered for the trailing arm style of suspension. So it can be done.
You will also need a new driveline as the housing on the 1/2 ton is shorter than on a 3/4 ton so a longer driveline will be necessary. So *if* you get an axle from a wrecking yard, get the driveline also.
You will also want the torsion bar assembly from a 60-62 half ton as well, if you are changing the rating.
It is almost easier to buy a rustbucket truck and put your cab/bed on it. We are working on a '66 1/2 ton that blew the rear end, figured easy swap was to 3/4. been stuck with too long drive lines, only driveline I have that would be for a 3/4 ton with 3-speed was out of a '61... the carrier bearing is in a different place so that driveline wouldn't work either.

On a '66 it is leaf rear and coil front so it is real easy to find replacements for. Coil rear and torsion front not quite so easy to find.