Hello, and welcome to the forum. I trust you, but I'm not curious at all why you're asking. Really.
I think your best bet would be to visit
http://www.oldbikebarn.com or
http://www.bikebandit.com, and do 2 searches on their parts finder. One for the stuff pertaining to the CB400, and the same for the CB750.
If the part numbers on that site match each other, then it's a safe bet they'll fit on both bikes. If not, then the next step would be to find a MC junkyard (or someone on a forum who has the parts you want) and try and match them up on your bike. This method will cost you some money, but you'll know for sure.
I also think you should try this question at
http://www.sohc4.net and see if you can get some feedback from them. We don't get a lot of CB400 traffic here.
Good luck, and keep us in the loop.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.