My '67, 3/4, steel bed, GMC came from the factory with a 4.56 Dana rear. I found 3.54 gears and posi carrier in a Dodge 3/4 auto, 440 and put them in my axle. Everything is the same except I had to find a different yoke because Dodge uses smaller U joints.
As longbox said, the parts are readily available and mostly interchangable.
As wrenchbender said, the GM 14 is a lot easier to set up.
Changing the 4.56 gears for 3.54 gears was the best thing I have done to this truck. The difference is night and day.
I have a SM456 4 spd, similar to your SM420 and a 292 six.
The second best thing I have done to it is the AC. P-steer and disc brakes rank 3rd and 4th.
Last Sunday I ran out across the desert to El Mirage at 70-75 mph with two bikes in the back to watch the land speed cars run.
With those gears and the other changes a trip like that is just as easy as with a newer truck.
