My 69 short step has about a thumb width clearance on inside and outside with 275 55 17 on 17x9 Legend wheel with 5" backspace. This is with the wider 71-72 5 lug axle. That's as big as will fit if you don't want rubbing and you are 7" down in the back.
On the front I have 17x8 with 4.5" backspace. 235 55 17 an I am REALLY happy with that combo, and my opinion is anything much bigger in the front looks funny.
Once again, opinions.... 17's are the smallest I would run on the front so the tie rod can tuck into the wheel which lets you go wider to the inside in the front. 20's really start to look a little odd I think. If I could get a good wheel for the price, 18's seem to be the sweet spot for tire availability.