Are you going to incorporate our billet snubber with the 100mm idlers? Or just swap out the stock steel 90mm idlers for our billet 100mm's? Is your blower pulley still 2.93? All of the above factor into different belt lengths.
With a stock upper and our 6# lower you would use a 685 belt. If you are looking to help with any sort of belt slip you need our snubber/idler bracket with the LFP replacement idler pullies. Pullies alone will not accomplish this. With the 2.93 upper the larger 100mm idlers will not work.