Originally posted by 48bigtrucks:
That rear diff probably has the same axles from the 40's well into the 80's.
Not so. There were many changes to these axle shafts over the years--different lengths, diameters, and splines. For the '51 2-speed rear, I believe the '46-'51 shafts should work (for right side), according to the '54 parts manual. The '46 and '47 ones must be for the splined-type, rather than the bolted-type, hub.
As far as swapping the entire 4100 rear into the 6400, I think it would work. The springs are the same size and the spring centers are the same. The 6400 2-speed driveshaft may be too short, though. It would also reduce the weight rating of the truck, of course.
