I have a Deere 48" tiller (very heavy) on my 430 (1987 diesel garden tractor, 20 hp Yanmar). The tractor handles the tiller well, but it is probably the maximum size I would want on this tractor. I use 6 42 lb, suitcase weights on the front of the tractor when using the tiller to help with balance.I have a 647 for my x748. It really doesn’t work it hard at all. If you buy a store brand, 48 inch or so, I’m sure it will be fine, but you will probably need to shorten the pto shaft. With the JD 647, you can make the dealer do it😁. I thought someone was trying to sell one here on the forum.
As with any other tractor/implement combination, you do not want to oversize the implement and lug (stress) the tractor. The 4' tiller covers wheel tracks when both rear wheels are in the narrow position (not good for stability so I leave them wide except when tilling).