Not sure how you squeezed 275's in the back of a stock 72 Swinger. I could just get 245's in, rolled fender lip, stock spring location. Had maybe 3/4" clearance ins'de and out.

That would just barely rub the leafs on hard turns.

Dr Diff says 1/2" more, which is maybe 13 mm. I guess if I was running 35 profiles instead of 60's and left 1/4" inside and out for clearance it could be done. Is you fender lip rolled or cut?

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.