rear end swap 56-66 Classic

59 six would be an AMC 15. 

There's a club up there that can help you find an axle. Rose City Rambler Club.
http://www.amcrc.com/RC/index.html
Mark in on the other coast, and has an AMC 20 axle -- you need an AMC 15.

Kinda expensive, but it's probably possible that you could have Moser or somebody make you custom axle shafts for it.

In the late 50s AMC used two different suppliers, and the axle shaft splines to the differential are different. THAT is the problem. Moser can indeed make a one-piece axle hub for you, about $300 for a pair. You can change the differential along with both axles and keep your housing, but then you have to set up the pinion gear depth to match the ring gear or you'll quickly wear the gears and create a howling sound. So it's not a simple bolt-in. The ring and pinion must be a matched set also. You'd be better off to change the entire rear axle. The 63-66 Classic axle should fit as well as 56-62 big Rambler axles. You will have to swap over the torque tube if you use a 62-66 axle. I'm pretty sure the splines on the pinion gears are all the same for interchangeability, AMC just neglected to specify the splines for the inner axle shaft ends. I'm fairly certain the hub ends are all the same. My guess is they figured the inner end wouldn't matter, the hub and torque tubes are the things that sometimes get replaced, and dealers could get the correct axle shaft for warranty work easy enough -- it's just not so easy 44 years later.

YES, only in big cars (Ambassador/Classic/Marlin... and 66 only Rebel, which was just a fancy Classic hardtop). The 63-66 TT is different than the 56-61. 62 is sort of up in the air -- it appears to be different from the earlier one, sort of like the 63-66 but with a different shaped front tube/transmission connection. Late model 62s may have the same connection as 63-66 models. The AMC 20 used behind V-8s and 232 sixes is the same no matter what the tube/trans connection is like.