Picked up a 2003 400EX with an XR650R engine.

[Blown 331]

One of the first ATC450Rs I ever saw was a 350X with CRF450 engine. It had the radiators mounted in the same location you'd normally see them on a dirt bike or ATC250R. The stock 350X shrouds would not fit though. Perhaps a whole 650R gas tank and shrouds could be used?
My cousin put an XR650L engine in a 350X. The 650L and 650R basically share nothing in common. The 650L is air cooled. That engine was too tall to fit in a 350X frame so he grafted the whole engine cradle out of the 650L into the 350X frame. That would not be possible with the 650R frame since it is aluminum. So I'd say you'd have to cut the engine cradle out of the 350X frame and make your own. Getting radiators on a 350X frame shouldn't be too hard but getting shrouds over them could take a little work. I did see a pic of an whole XR650R converted to a 3 wheeler. It basically just the dirt bike with an ATV swing arm and 3-wheeler front end, it was rideable but needed a lot of work yet in my opinion.
And from what I could find unless you are running the lean stock jetting theres really no overheating issues with the stock radiators. They are quite huge, bigger than the MX bikes. In my opinion the XR650R is just in a league of its own, nothing else like it.

[oldskool83]

suspension and chassis upgrades get costly quick is not your not keeping it don't do it. a plus 2 swinger would prob be better idea if you want to keep the front end down.

[Blown 331]

This is definitely a keeper but I'm still not really planning on suspension modifications. Swingarm maybe something to consider but that would be about it.

[SteveCZ28]

after seeing the side pic with the ful front fenders on it again, it looks sweet. most people wouldnt notice that motor in there, so it just looks like another every day 400ex. but those that have the trained eye will notice it. id love to have one of these. looks like a blast