The engine was an extremely sound unit, i ran it in my 600 chassis for 6 months while i was refurbing the 600 lump, and to be honest, with the single 40mm dellorto and open(ish) pipes it's performance wasn't that much different to the std 600, obviously a bit less acceleration, and slightly less top speed, but the 400 engine seems to rev more freely than the 600, with less vibe.
prior to selling it i honed the barrel, and fitted new rings. and as far as i can remember the head was in great condition....
anyway, good luck with the project