Biggest tire on 1969 dart

Biggest I could get under my stock located s/s spring 69 Swinger were 255/60/15s on 15x7 w 4.25 backspacing. Not much room either way. have 1/2" between tire and wheel lip, 1" between tire and springs. With your springs being moved in a little, I would guess a 15x8 would fit w about 4.5 bs. I tried 15x8s w 4.5 on my car and they were about touching my stock located springs, so I could not use them. I was lucky to have a buddy w assortment of rims to try.