If you're talking about the whole rearend, they are a bolt in swap out. The original drive shaft and coupler that pins onto the rearend can also be used. One big difference may be internal versus external brakes.
I just swapped a narrow frame rearend into my 800. The 800's original driveshaft and coupler fit like a glove. They were both internal brakes, so a bolt, fluid change, and go swap.
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Josh
Diesel Cub Cadets...........
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