Because its a racer and it is not allowed to have more than 350ccm.....
Ahh ok then. I seem to remember that the oil passages don't line up so swapping the jugs isn't a simple thing. I also seem to remember the transmissions not being swappable either due to size differences.
So, think outside of the box a little, if what bodi says is true, that both have 50mm stroke, it might be possible (probably not cheapest) to re-sleeve the 400 with jugs to fit a 350 piston??? Would need to make sure everything lined up of course