Tim first welcome to WFM. The pulley size is 3 inches. The belt I don't think anyone can help you with without knowing how far and which direction you moved the original crankshaft centerline from the stock location. I put a Briggs 675 series in my 70 and the belt was 2 inches longer than stock. That doesn't mean yours will be though. What engine did you put in? Where is the engine mounted? Maybe some pictures? If the crankshaft centerline is in the same place just buy a replacement JD belt. Roger