yes it will fit with a few modifications. the front drive shalf or a warn 424 from a 450 will have to be used, the rear drive shalf ujoint will have to cut down about 1/4" and u will have to use the 400 top motor mount. I think the 450 CDI box is also different than the 400's, so this will be needed as well. But all the wiring should hook up with no problems. This is the set up that I have on my bike and have not had any problems. I have everything from a 450 on a 400 frame, motor, a-arms, axles, shocks, wiring, etc.