just looking at tthe front wheel. the caliper bolts have the same spacing between them and theres only about 5mm difference in the width betwwen fork bottoms... the disc is about 300 dia.

as for the rear, the 660 swingarm is about 100mm longer than the earlier xt250. the width between swingarm (axle) ends is about 20mm wider on the 660.
where the swingarm bolts to the frame the 660 is about 55mm wider than the 250. the zx swingarm had about the same differences. so swapping swingarms is out of the question.

im really hoping to have a set of mags for road use, that can be easily swapped to the standard R wheels when needed. next I'll check the axle diameters for compatability...

I once fitted a PE250 swingarm and yz forks to an xt500 back in about 1986, so i guess I got the 'swap' thing happening way back then!