Hi Ryan. I don't think that JD technically made this for 318 tractors, the #4 was made from 1981-1983 and did have a mounting option for the older, closed rear frame 300 series tractors (300, 312, 314, 316 Kohler and 317). The parts book lists two drive belts, one for all but the 317 and another for the 317. With the right mount adapters and hydraulic pump kits this also fit 200-216 (and flat fender 110-112) tractors as well as the 400 tractor.
It would probably work on a 318 as long as the extended front bumper a 318 has doesn't hit anything on the splitter frame and if you could find the right length drive belt.
Kent

It would probably work on a 318 as long as the extended front bumper a 318 has doesn't hit anything on the splitter frame and if you could find the right length drive belt.
Kent