Greenman,
Any large body pwk carb should fit fine, the carb that I am working with is a 39mm that was originally off of a kx 500. It is not stock as I have had it modified by Ron at RB Design and have done some extra work to it myself. I still have my stock carb, that has also be reworked so I can always put it back on if I need the added low end responce. I would be sure and talk to Eric first if he is going to do your motor work and get his input. For good results you must build your motor as a package or you will just end up with frustration. The larger carb is a top end mod which should in most cases be used with other mods that are geared in the same direction. Putting a big carb on a bike that is in other ways setup for all around power might just end up losing power everywhere. I would be glad to help with your jetting if you go the big carb route, my experience is that the larger the carb the harder it is to dial in the jetting. If the thought of spending the day jetting instead of ridding doesn't make you sqeamish then it could be well worth your while, but if it does you would be better off going with a bored out stock carb as you have much more data to work with. Right now my jetting is a 50 pilot/ 178 main/ #7 slide/ NOZH #3 with HP2 at 32-1 mixed with 110 Trick fuel in my kdx. The big carb is also a tight fit so jetting it is a bit more of a hassle. I will e-mail you a couple of pics that compare my 2 carbs. Good luck with your quest for more HP:) --Dan