can I mount 20x10x10 paddles (on Douglas .125 or .160) without any problems for the dunes on my 400? the stock size rear is 22x11x10.
I can get a good deal on Cheng shin dune sliders. Are these ok?

 

You can do that, but I think you would be better off with a larger tire. My buddy was running a stock Tblzr 400 with some 20" sand sharks. That worked fine until he piped and clutched his quad. After that he was burrying them all the time. He found an used set of 22" Edges and was much happier. Kenda has a decent paddle in a 22" for a good price. ITP Sand Stars are a low cost 22" as well.

 

I ran 20'' paddle masters when i had my 500 and they were terrible. i ran 22-11-8 dune hoppers on my 400 and they worked ok. now i have 21-12-8 haulers that area actually taller than the 22''s i had ill try them for the first time this weekend.