1967 Barracuda. what are largest front and rear tires i can use with my 15x7 sbp wheels? 4" backspace?

Hi folks, trying to find out the largest tires i can use front and rear on my 1967 Barracuda. I have 15x7 SBP US Mag wheels, with a 4" backspace. Car has stock suspension with drum brakes. Thanks much