You can use either as both will work. If you don't have any issues with belt slip, the 90mm sizes will be perfect and you will be guaranteed to use the same belt. I don't exactly know how many 90mm idlers are on a lightning, so you'll have to look or maybe someone can chime in. Thanks
Badcobra, we dont replace them all, like you guys do(Although ive been thinking about it, just for looks). Our blower belt is separate. 90s for stock pulleys, and 100s for pulley swaps. Im running a 6# lower, and the old style Metco Aux idler bracket with (2) 100mms.