I honestly wouldn't change the tank at all, I love that style of tank. I'd like to have one for my CB350. I'd change the seat a bit to work with the tank instead.
If you do want to change the tank it may be hard to find one since the CB400 has such a wide frame in that area. I think the CB450K has a wide tunnel under the tank but the section where it connects to the frame is narrow though. A tank from an earlier CB400T would fit as well as a CM400 I believe.
I'd go for something like this: