LMW1970:
The tank sits fine right where the old one did but the bolt for it doesn't mount up. You have a few choices here. A) You craft a bracket which bolts up to your upper engine mounts, you will have to get longer bolts for this to work. B) You craft a bracket which bolts up where the radiator meets the frame. C) You just let the gas tank sit where it should and don't bother with the bolt, which is what I did. Some people don't like this but in my experience it's perfectly sturdy. Once you put the seat on it locks it down pretty tight and after bolting the shrouds on which connect to the radiator, it's not going anywhere. I've been riding it this way all summer doing supermoto, motocross, and some pretty rough trail riding with no issues, so this is my suggestion, plus it's easiest. As for the seat, it is a stock seat, but one for an 03-05 yz450. Pretty easy to find a used one on ebay. The only really difficult parts of the swap and really they were that difficult, just took a little time. One was the subframe, the parts that bolt to your frame are a little too wide, so you have to grind them down a little. There's info on doing this in other forums. And the shrouds, due to WR's using larger radiators, on the inner part of the shroud you have to sand or cut some of the inner plastic down so that they clear the radiator. Overall it was a pretty painless job and I'm extremely happy with the results.
crash279:
I think the XR650R would be an amazing supermoto bike, especially if you plan to use it on the street as well. I've considered this for a hooligan/canyon carver style ride. There's also info I saw somewhere about swapping out the forks for some CR450 ones. Gave the bike a pretty mean look IMHO.