Nope. The 420AT is fuel injected and has, IIRC, 14 electrical plugs on the motor. Wiring will be different, frame will be different, driveshafts won't be in the right place, etc etc.
I guess you COULD do it, but the cost would probably be more than MSRP on a new DCT Rancher.
Replace the clutches on your 400 and keep riding it. If you've gotten 4K miles out of the original clutches just put new ones in and go another 4K miles.
Now you MIGHT be able to swap a 350 Rancher footshift motor into that 400 without too much trouble. Still won't be bolt in, but would be much more likely to be workable. You'd still have to replace all the wiring and electronic crap with 350 stuff though.
If you really love the machine, and all you're worried about is the clutch, I wouldn't worry about it. With the 400AT there is only the centrifugal clutch. No shift clutch in these. You're only in for $200 and that includes a new one-way and outer clutch basket, if you can handle doing it yourself.
Now if you can't handle doing it yourself, it can run up to $700 at a dealer. In that case, I'd dump it like others are saying...
It would be one heck of an achievement if you could pull that mod off I have to admit, the fabrication side of it wouldn't be so bad but the wiring....?
If you go for the engine swap, start a thread, is be interested in seeing that.
Top and bottom of it is, I think (as others do) that you'd be better off replacing clutches.