For the record, remember there is no "chain thing" on a shaft drive 250EX!

Clearance from the frame will not be a problem of any kind! Being radial, the SR's should give a nice ride! I have the ITP TrakStar radial on our 250's right now, and i bet they probably use the same casing. You will lose a little ground clearance and top speed, because they are shorter than the stock (22" front/22" rear) tires, but you will gain some low end power.

http://www.itptires.com/sportatv/holeshotsr.html

If I were doing the rear with a radial, I would probably also want a radial on the front. Note that the Baja rims are wider than stock and have a greater offset. This makes the quad several inches wider, for a nice increase in stability...but again I would not do the rear without also doing the front!

If you don't have money for the fronts as well, if it were me I would stick with the stock rims and just get new SR's all the way around.