yes, i did have the 450r shock and it was great, and yes, kens bushings work exactly the same with it as the 400ex, and also used them on the outside of the stock mounts with a 250r shock on my sons bike,
but the 450r requires you to cut off the front fender tab and its matching metal frame mount on the kicker side, imo....the 400ex is the same and easier to mount... no cutting of anything
but i just sold my 450r and got myself a "400ex works ultracross" and will be using kens bushings for it also