I have the same brand and size tires on my 68 Convertable and use airshocks to get them not to rub. A temp fix at best.

I am using the air shocks until I replace the rear springs this summer. As far as I have reasearched, this is the safest most reliable way to get the larger tires to fit without rubbing or bottoming out.

As for the others who run this setup without problems, I guess their springs haven't sagged as much as ours pover the years.