If you do 2 inch drop spindles in the front, make sure they have 0-4.5" backspacing. I have some and started with H2 wheels. NOT a good idea. The backspacing is too much, 5.375 I believe. Im not sure on backspacing for 18's but from test fitment, I'd imagine max would be about 4.75". If you went with 20's I know it wouldnt matter cuz they will clear.
Its the lower ball joint bolt and nut that cause a problem with the backspacing of the wheel.
Truck with 2 inch drop spindle and Torsion bars backed out. H2 wheels 17x8.5 w/ 5.375 inch backspacing.
Truck with same 2 inch drop spindle and torsion bars backed out. Cragar soft 8 17x8 w/ 4.5 inch backspacing.