Here is what you need to put a 280 300 tire on your 1991 1999 Softail and may fit on some custom frames.
Wide right side drive swingarm allows use of a 280 300 rear tire with a 1 1 2 belt.
Swingarm is 2 longer than stock Softail section and includes adjuster and 1 axle. Use with 70 tooth pulley, strutless fender, fender tabs, 136 tooth belt, and weld on caliper mount to allow use of a 280 300 tire.