I found a guy that had it on CL for i think 80 bucks. He had taken a cheep 250 ninja, made the motor into a rail and had the empty frame sitting around. There are a few of these builds if you google search 250 ninja and CB400 they pop up. I think there are several on this site. Honestly i decided to go this route because I wanted a cheep build and the original old front end, front and rear rim/hub were in horrible condition and i thought it would be fun.
Here is one of the builds http://www.dotheton.com/forum/index.php?topic=29721.0
Yep, whole tripple bolted right on, I put new bearings in and milled down one of the old bearings to use as a shim, 3/16 gap if i recall. Rear tire, I used all the ninja spacers but the original axel and chain tensioner assembly.