back spacing for 90' gmc 1/2 ton

i have a 1990 gmc 1/2 ton pick up. its 2 wheel drive. im about to by 20in rims to put on it. i have a 4"-6" drop on it and plan to bag it in the future. what is the best size rim to put on my truck so it doesnt hit the fender. what backsizing do i need? and what width would be good? 8-8.5? more? please help