tucking 20's on 85 c-10

what would i need to do to tuck 20 x 8.5 rims on an 85 c 10 with a 5 inch drop, not sure what tires i will use nothing too thin though, any suggestions on what size tire would be nice too

255/35/r20! if ur just lowering it it wont tuck in the front only in the back