http://WWW.CONVERSIONPARTS500.COM
I have not used any of their products but from what I see, they do quality builds as well as offer parts for the do-it-yourself-ers....
First, let's take a look at their manuals..
http://www.conversionparts500.com/manuals.php
They have developed several manuals for different aspects of the builds, Nice!
Let's look at the 2002-2007 CR250 frame manual.
It is 15 pages of clear and concise technical info with pictures, measurements, ect that should make this build easy.
I love their "Y" piece..
You can buy a 03 CR250 converted chassis for $1200.00
http://www.conversionparts500.com/#!/~/ ... ort=normal

Having been working on my KX500 Conversion for a while I can tell you that this is a nice way to go.
Buy it, and bolt on your stuff.....
The only negative I see is that it comes with a clean VIN and bill of sale, no title
That will really hurt sales in some states.
Just browsing around I was looking at their parts for a 04-12 CRF250X conversion.
They even offer a modified gas tank...
http://www.conversionparts500.com/#!/~/ ... ort=normal

They also offer kits:
http://www.conversionparts500.com/#!/~/ ... d=21887933

In conclusion, if your in to riding a big bore 2T Honda's but want the updated chassis with more modern suspension and brakes this wouldn't be a bad place to start...