max wheel / tire on a step side

[Stevemoore]

Hey folks, kinda new here and I could probably search tis topic, but wanted to throw it out here anyway. Trying to find out the widest tire and wheel that can be put under a 68-72 step side fender. No tubs or and other mods. I'm wanting to go with 20" wheels. Truck is probably 6 inches lower than stock in rear and 4 inches lower in front. Probably going with 18" in front and I want to fill the wheel openings.