When selecting a belt for a torque converter, the outside diameter of the belt is not the best way to measure.
Take a measurement from the centerline of the crankshaft to the centerline of the rear driven pulley's shaft. This measurement needs to be within 1/16"! If you can provide us with that measurement and the style of torque converter you have, we can get you the correct belt.
There are two styles of torque converter belts. One is the symmetrical belt that has an equal bevel on each side. The other style is the asymmetrical belt where one side is nearly flat and one side is beveled. For this style belt, make sure the flat edge goes towards the engine side of the clutch.