Hee ya go:
I'm finishing up a build with a CB400F engine in a CB350F frame. Yes, they're compatible. I like the CB400F front pegs much better; they're set back about 6 inches. Hate to give this away, but they will also work on 550s and I'm guessing 750s, with some minor mods, for a "semi-rear set" kind of deal. I guess since I'm sitting on 3 sets I'm good for now. The rear pegs, however, I like much better on a CB350F as they are mounted to the frame. On the 400F, they are mounted to the swing arm. WTF. If you switch over to 400F pegs you'll need the shift linkage (obviously) but also the rear brake lever. On the left side, the new shifter with linkage will interfere with the side stand, but not if you cut off the little ring of metal that comes on them; the main part of the stand is fine. On the CB350F, the mount for the brake lever (a small tube welded on to the frame) sticks out about 1 or 1.5 inches more than on the 400F, so if you mount a 400F brake lever on a 350F frame, it will stick out a bit from the engine. I used a grinder with a cutting wheel to cut this off, and stuck some of what I cut off on the brake lever behind the mounting tab (essentially making it more like the CB400F). Takes a little finangling for the brake light switch, if I remember correctly, but nothing major. Tanks fit just fine. Side covers are, obviously, different, but I am not running side covers. Seats are different. Handlebars on a 400F had a rise of about 2 inches, and on a CB350F the rise was about 4-5 inches. As far as I know, there was no stock 4-1 exhaust for the CB350F; the CB400F is recognized as the first stock 4-1 exhaust system (someone correct me if I am wrong here). That being said, a stock CB400F 4-1 exhaust bolts right up to the CB350F engine, although you can't use stock CB350F pegs and need an exhaust mount in the rear. Key is mounted differently (as are idiot lights). The CB400F has a really ugly headlight and front turn signal mount, and the CB350F has cool looking headlight ears and gaiters for the front forks. Forks are same diameter (33mm) and are interchangeable. CB400F has 6 gears. I've found with both that keeping stock exhaust and switching to pods doesn't require rejetting, just tuning, with both bikes. I will let you know what happens with my new bike when it's up and running -- I am running custom 4-4 exhaust and pods. Different front fenders on the CB400F and CB350F, too. Oh yeah, you can turn the headlight off on the CB350F, but not on the CB400F; it's always on when the bike is running. Different horn, too; CB350 is cooler and covered in chrome. I guess that's all I can think of now...
I know a ton of people slag off the CB350F when compared to the CB400F, but I honestly like the CB350F styling better (cooler tank with knee cuts, cooler seat, rear pegs). And I gotta say, I've had a CB350F for about 15 years. Rebuilt the engine back then, and with the occasional carb overhaul and annual tune-ups (and a slightly rattling cam chain...) the thing runs super smooth. I hold 75 easily on the freeway, which isn't too bad, but it'll go faster (I've done the ton on one of my old CB400F bikes, however).
Recent photo here: http://wordsofwezdom.blogspot.com/2012/08/cb350f400f-update.html
