Finding a used one out of an American could be tough, I would tend to think that not too many have been parted out yet. 

The unit being the same as a 1655 or 2-70 is only partially right. They did use a chain coupler, so the majority of the unit will work. The input shaft, front cover and bell housing are different on an American, since it bolts up to a Cummins engine. The good news is that the Cummins used a SAE bell housing, and so did some other White tractors, namely the 2-105 and 2-85 with the Perkins engine. You wouldn't be able to use one of those units without changing the back of it, because it did not use the chain coupler, but instead used a trunion mount.

Your best bet is to get a over/under out of a  1750/1850/1950-T or 1755/1855/1955. They use the same input shaft as the American, and have the chain coupler on the back side. You can use the bell housing that is on your American, just remove the bolt-in shaft support, and bolt the bell housing to the front of the over/under. The only challenge will be hooking something up to the shift lever so you can make it shift. I'd recommend a flexible cable like is used for the throttle linkage on the American, but a guy might be able to rig up some kind of rod that you can push/pull. You'll also need to put an oil cooler in front of the radiator, and lines running back to the over/under unit. You might get lucky, as the parts book doesn't show a separate cooler for the over/under, it was combined with the hydraulic cooler. Look up front on the lower right corner, if there are 2 cooler lines that are capped off, then you are in luck. If the oil cooler isn't close to the width of the radiator, then you'll have to find a cooler to fit in there.

When I rebuild over/unders, I take a piece of radiator hose and clamp it on the chuck of my 1/2&quot; drive drill and the other end on the input shaft of the over/under. We took the cooler lines from a salvage tractor and cut them to just a few inches long, clamped a piece of clear plastic hose between the two, and hook that up to the cooler circuit. The clear hose gives you an idea of how well the oil is flowing through the cooler circuit, and therefore the pump, as the pump has to generate pressure for the cooler circuit to open up. Once that is all hooked up, we spin it with the drill. It places a good load on the drill, so you'll want a good sized one, and to not run it but for short bursts of 10-20 seconds so you don't overheat your drill. You'll still be able to run it long enough to shift it through all of the gears, and will notice the drill bogging down with each shift up, as each range takes more power to run than the one below it. You could also put a pressure gauge on the side cover to see what kind of pressure you are generating, but it could be on the low side since a drill isn't going to spin it up to the speed the engine will run it. But it should still be enough to tell if it is shifting and how quickly.

Another quick test would be to spin it up in over drive, and then stop the drill, and quickly try to turn the sprocket on the back backwards. When an over/under is in over drive, it can not turn backwards, and it typically takes a couple of seconds for the pressure to bleed off enough to release the over drive clutch pack. This is why it is hard to shift the main transmission if the over/under is in over drive. The longer it takes for over drive to bleed off the pressure and allow the unit to turn backwards, the better the seals are in there.

Of course, none of this is as good as having the unit in a tractor where you can test it against a load. The sprag clutch could be weak, but it wouldn't show without a load against it. If I were doing it, and didn't know the history of the over/under I was putting in, I'd open it up and put a seal kit in, and inspect the parts while I am at it. Seal kits are a little over $200, and there would be a few hours of labor if you have to have it done. If you have a shop manual and are good at following directions, they're not too bad to do yourself.