barspin clearance has to do more with axle to crown length (rake too). a longer axle to crown will let you barspins with fatter tires. your best bet to barspin on a 700c meant for would be to look for a "straight bladed" fork. otherwise 400a-c 26" forks will work.

what size tires are you running now? if not 23c those would probably be your best bet to barspin without clipping your downtube.