I have a '59 3600 Fleetside. Original gears were 4:56, which made highway driving impossible. Transmission is the SM420 4 speed, but the granny gear means that it's really a 3 speed unless you're pulling a stump. I replaced my rear end with a Dana 60 from an '87 Dodge 2500. Limited slip with 3:54 gears. Had to widen the spring perches and buy a new yoke for the U-joint, but other than that, it was a bolt-in operation. I can now cruise with ease at 70+, although my truck is an original V8 truck with a 350 in it.
Chris
