the rear has quite a bit of room inside for big tires. since you're going for a functional drag setup, you should run a pretty high offset, lke 15x10 +40 or something like that. check the
max rubber thread and
use a calculator to compare it with your old tires.
you'll loose a bit of gearing with the smaller tires, but with the better compound and tire construction you might be ok. some of the guys on ka-t.org are really into drag racing. they can probably help you choose a size better than i could.
note for those reading, according to MT's website, the "10" is the rim width. the cross section is 11.6 in this case. the 26x10x15 is roughly equivilant to a 295/47/15.