1975 XS500 Tires

Ive been shopping for tires for a 1975 XS500. It's been a real pain trying to find matching tires that fit (3.25-19 and 4.00-18). I got a list of a few i like that should fit, but here is my question: im measuring the rear rim to be 3.00 inches from bead to bead, some of the tires I like recommend a rim size of 2.15. How much of an issue will I run into with that fitment? Other than the rear tire sitting really flat, would this be a problem?