Like yettiatcpg, I also run Bi and Tri-claws on my 650. They clear perfectly in all aspects on stock rims.
Mine are 27x9.75x12 and 27x12x12. You can see pictures of it at the url below. You can go with 28x12.50x12 on the rear, but I wouldn't go with that size on the front. You'll need spacers I believe, I could be mistaken but I think they'll rub without spacers on the front if you go with 12.50's on the front. Also, I'm not so sure those will fit on stock 650 front rims.

Prairie 650 Pictures

 

Zbop I have 27" MUDLITES installed no problems with rubbing
Go big or GO HOME !!!!!!