More likely, its production tolerance. There are many reasons why, but a good measurement of the clearance to the fender is in order. On my 96 Impala SS, the rear axle is shifted to the right by a complete 1.5". I don't have rubbing since its the rear axle, but if I want that sexy 315mm rubber I'll have to run a different offset on the left and right.
Its also possible that the fender was dented and repaired with a slightly different contour, or that the alignment is different on the right than the left. Often times alignment shops will put different camber on each side to help correct for crowned roads. Its also possible that caster is affecting the steering and camber gains in the turn.
Big wheels and big rubber mean big measuring. I'm sure its not the spring. Even if it endured 50 years of sagging, the spring may have a different installed height, but not a different rate.
There are a thousand reasons why one side could rub and the other be fine, but I'm sure the spring is not one of them.