He has a backspace of 5.75 inches. Plus 1/2 inch added to that for the tire equals 6.25 inches. His available space from the wheel mounting surface to the sheet metal is 6.5 inches. The tire would be 1/4 inch away. Not good. At least an inch on the inside would be better. You could get away with 1/4 inch on the outside if side to side body movement is controlled. Only way to mount a tire that adds no sidewall to the width is with a very narrow tire like a 195 or 205. You'd still be 3/4 inch from the metal. Not recommended for a narrow tire like that on an 8 inch rim. Sidewall failure in short order. Correct backspacing takes in to account tire cross section too!