I use a 48" Millonzi on my DK40se and it does a great job. The Markham is very similar in design and construction though a bit heavier so that is the one I'd buy today since Millonzi went belly up.
If it were me I'd get them to build the top grapple about 1/2 the width of the grapple (ie 24 inches) any wider and I'd be concerned about twisting the upper grapple with an asymmetric load). That is wider than my Millonzi (about 18 inches I recall). I'd also have them put in extra spacer tines so the gap between bottom tines is about 5 inches to keep smaller stuff from falling out. My Millonzi has and inter tine distance of about 9 inches. I wish Markham used the same "rocker" bottom that Millonzi did as it allows you to use the weight of the tractor combined with curl to leverage roots out. I haven't used a straight bottom grapple so I don't really know for sure the rocker is better.
I recall people posting the that Markham weighs about 400lbs. The Millonzi is advertized as weighing 280lbs but I'm a little doubtful it is that light after looking at the Markham in comparison and after trying to muscle my grapple around. I suspect they are pretty close in weight so the Markham would be just perfect for a DK40se/KL401.
How are you going to control the grapple? The rear remote system works fine for grapples though the controller for the DK40se is just a tad lower than I'd like. I have a diverter valve but I'd probably do either the rear remote with a 3 inch extension on the lever or go whole hog and get an electrohydraulic switch next time.