is it in really good shape? that'd be hard to do, unless i felt my skills were really up to doing it properly.
my 400 looked and ran like crap when i started out, so it was pretty easy to hit it with the grinders, but if it looked nice, i would've been nervous.
i guess it's all in the confidence you have in your skills. have you done similar work before? hows the paint and seat look now? i wouldn't consider the millage/engine condition that much, unless you're chosing between chopping/selling. it'd be pretty nice to build something around an engine that i knew had a ton of life left in it.
that said, we just got a POS 550 and bobbing it out was my first thought.....the more i look at it though, i think i'd rather see it fairly stock and bob the 400twin.