UPDATE ...
You can fit these 12.5s, but it takes a bit of tinkering ... and see my post "failure is Imminent" I wouldn't recommend this setup with the stock axle. I'm going to guinea pig it
....
What I did was flip the rim, gained about 1/8th inch doing that ..
Added two thick washers between the axle seat and hub and gained about 1/4"
Added 1 thin washer onto the wheel bolt lubs and gained 1/16th.
Total gain of about 1/2 inch between the tire and rear frame corner
that little bit of movement, plus the added tire weight, width and traction, i'm pretty sure is going to be the end of the stock axle very quickly.
Just going to be easing into the gas off the start as much as possible.
As far as mounting these, dropping the first side of the rim in was no problem, getting the second bead in was a real battle as i don't weigh enough to compress the tread outwards when standing on the tire to get the bead under the bead seat on the rim. I started chewing up and stretching the bead with my bars until i coated them and the bead with a LOT of straight dish detergent ...which made it all the more fun lol ...