88-98 Wheel/Tire Size

Hey guys. On a bodydropped 1990 Reg Cab Short Bed Chevy, right now im running an 18" wheel with a 255/55/18 tire and i believe the tires are too big and or wheels have wrong offset. What i want to know is we want to upgrade to a 20" wheel and go down to a lower profile tire. What offset 20" wheel and what tire size will tuck on a bodydropped OBS and not rub?

I am running a 20x8.5 wheel with +14mm offest and a 255/35R20 but I want to run a 245/40R20 tire now.

i'm running a 20x9 with a +14 offset i think and 255/35/20 tire