Funny, first thing I noticed from the pics is that there are no dings in the tank
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That will be handy when you get around to paint. That's definately a 400F clutch cover and as 333 said, drums did not come of 350F's. To me, that looks like a front end off of a 350 twin or similar. Right from the wheel to the guages. Maybe a CL model, given the high handle bar. The exhaust is made from the stock headers for the 350F with custom pipes welded on below the kicker. Hows' it sound?
Now, If you want to know if its a 400 engine or not, look on the front of the cylinders below the right two exhaust pipes. There is a place there that states the displacement. A 350F says 347cc and I beleive a 400F should say 408cc?
Also, like aptech77 said, the 400F's had a six speed tranny. The 350F a five.
Matt.