I run 35's on my 99 XJ with a 3.5" RE Super-Flex kit, RE .75" coil spacers, RE .75" shackle, fenders trimmed and plenty of bumpstops with no trouble at all.
Drive the Jeep up on to a trailer ramp or RTI ramp until the compressed tires are about to hit the fenders. Measure the distance from the bottom of the existing bumpstop to the top of the axle where it is supposed to hit and then add an inch to account for the bumpstop compressing. That is how much bumpstop you should need. Here is a great write-up I used to bumpstop mine. Good luck!
http://www.rocklizardfabrications.com/Adjusting_Bumpstops.htm