Vehicle: 1971 cutlass, has Hotchkis tubular front upper and lower A arms with 1" hotchkis Big Block drop springs and extreme sway bar up front.

I would like either a 245/45/18 or a 255/40/18, what are the recomended wheel backspace specs for this size tire/suspension?

I found a wheel/tire combo of 245/45/18x8.5" 25mm offset with a 5.7" BackSpace, will this size rub the fenders/frame?

I found this cool database and punched in these numbers, does these numbers sound correct? should I lower my offset to 19mm and BS to 5.5"?

http://www.rimsntires.com/specs.jsp