I built one over the summer with a 800 VES twin SLP heads, SLP twins, Slp porting Stock 440 air box, 19/ 39 gearing 146 Assault rear skid, camoplast 2 inch challenger track worked SUPER well. I sold it earlier this winter.
As a rider forward chassis is hard to say. It feels NOTHING like any rider forward A arm chassis you can definitely tell it has trailing arms. You can definately tell though that your center of gravity is in a rider forward position as the sled is a breeze to ride. The front end when set up correctly floats unbelievably well. It sure feels a lot different that even a Pro X 2 does of that year with similar mods as I had a 04 Pro X 2 144 800 mod as well and you can definitely tell that your center of gravity is different on that sled.
The engine also sits in the XR race chassis much different than it does in the consumer chassis Pro X or X 2 or even the older 440 chassis sleds including the 03 XR. Its tipped slightly forward and sits very low in the chassis. IMO if your going to run this chassis as a mountain sled you CANNOT use the stock trencher special skid, the skid couples way to much and to quickly to ever be used as a mountain skid. And coupling = trenching.
There are several tricks to getting a 700 or 800 in this chassis and things that you need to address before hand, and if you do them correctly you can run oil injection on the sled and be pull and go. Its all a tight fit as the hood and Pan are 6 inches shorter than a consumer edge or Pro X hood and pan but if you know what your doing and what to order it works out fine.
Also to answer the above questions the 04 Pro XR 800 (orange sled) is a 03 Pro XR race chassis with the under the engine steering... Not a rider forward sled or the at all the same as a 04 PRO XR 440 chassis. The 04 PRO XR is a one off chassis. Nothing was ever made like it before or after. And Parts for it especially hoods and windshields are EXTREMELY hard to find as the Hood is Shorter thinner and cut WAY different than any other Pro X hood As is the belly pan. Now you can cut your own out of other models but they do not fit the same or look near as good. Most of the parts have been discontinued form Polaris as well. So if you find one make sure all of its there
