Lowering,10" Rally's, and tire size?

[slam59]

I am still trying to decide how low i wanna go with my drop on my 74 squarebody. I currently have 15" wheels on,10" rallys on the rear, and 8" rallys in the front. Tires in front are 255x60x15 and the rears are 275x60x15. I look like the look of a wide tire all way round and was wondering if I put 275x60x15's all way around, what would be the lowest front and rear drop I could go with out rubbing or other issues?