Rear tire clearance?

Just how much clearance is needed between rear springs and tires? On my 73 duster with 275/60 BFG's and 4.5 backspace 8"wide wheels I only have 1/2" clearance. Seems awfully close. Just did a test fit haven't driven on them. Any chance they'll rub under different driving situations (cornering, braking etc.) I don't want something that isn't safe.