Rear Tire Rub

[bulldog19]

I have 68 Camaro Convertible Hotchkis TVSS suspension and the rear tires rub the fender lips when going over the slighest bump/dip in the road @ speed. Tire/wheel combo is 17" rim with 285/40 tire. I have roughly 3/4" space between top of tire and fender trim. I have a decent sound system in trunk, 2-10" subs and an amp which i know has led to this problem.
Future plans include going to 18" rim/tire but that's not for another year or so till I can save the $$ to do so.
Front i have no issues combo is 235/40 on 17" gap there is about 1-3/4" but i would like to lower here. Suggestions? Car has been done over a year and i have about 1500 miles on it so I thhink I have experienced all the settling that will happen.