I ran a 1964/65 truck 12-bolt truck rear end (they are different from the car 12-bolt) in my '54 3100 for about 10 years when I switched over to an open driveline. I had to take off the coil spring stuff, weld on the leaf spring pad, and have a drive shaft from a newer truck shortned, but the rear end fit just fine. I don't remember the measurements, but there was no discernable difference in looks.
