Well, we've made some progress, but its quite frustrating as there's no evidence of anyone having done this before.
First up
the b250 engine is an inch narrower than the 400, which neccessetated some shaving of the engine block and will require some welding on the frame.
I have 3 sets of carbs in various states, new emusion tubes for said carbs thanks to jay (and no thansk to usps), a spare cdi, a cdi that needs capacitors, a gsxr swingarm, a rf900 rear shock (thanks again jay), gsxr front forks.
I need to get a factorypro stage3 jetkit (for the needles, jets and pod filters) and some free time and i should be good to go.
here are some progress pics
if i ever get this done, i hope to try and get the 250 engine onto this go kart