Help with tire size

[SPRNTRCR2]

I have a 67 stepper that I am about to do a disk conversion and a 4/6 drop. I am walking a fine line with this drop because my driveway is about 1/10 mile long and gravel. I went with it vs the 3.5/5 simply because I wanted the stance a little lower than the 5 in the back. I am looking at going with ridler 695 wheels (8 in the front 9.5 in the back and 18 tall). But the other night I had a brain fart and thought I might get a little more ground clearance going with 20's all the way around. Do yall have any tire size recomendations for the 18's and the 20's that will still work good with the stepper? If I go with 20's I really want to go 10 in the back but should I stick with 8's all the way around. And I really dont want rubber bands for tires either. I still have the shorter stock rear end.

Thanks in advance