Not trying to be a smart@ss or anything, but if you are worried about saving a few bucks at the pump, why are you going to spend $2,500 on a tranny in order to gain fuel mileage? I think it would take YEARS for the upgrade to pay for itself. However, if you want to make the swap just for the heck of it, I say GO FOR IT!
I agree with alot of the others though, I would shop around a little while for a salvage yard/rebuild deal before I dropped that kind of cash for a "kit". Just my $0.02