AFAIK, 3/4 ton and up are open driveline starting in '50 or so..........it's the 1/2 tonners that have enclosed up to '54-'55 1st (some '55 1st are open).
Rear gears should be 4.57's, but may also be the dreaded 5.14's
.
The newer third member should bolt-up; the u-joint size may be different, I am not sure (I may be thinking _Ford here).
To make sure about the ratio currently installed, try this:
"Jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full
revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 & 3/4 turns, you probably have a 3.73 gear ratio. Turning the tire for twice the
number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading."
(quoted from West Coast Differentials)
