In that case it is an F engine and you may just need the correct carbs, all the ones i said before will work but may have jet differences which can be changed, obviously a correct set for the model would be best.
I would recommend new rubber inlet manifolds and carb to air box rubbers, the latter are ok pattern as they only come with plenum from honda.
When collecting bits try to get mounting brackets as well if you havent got them.
I have a spare 550 filter box and may have a plenum without rubbers to spare.
Post pictures of what you have and we can help as much as possible.
Sounds like somebody just picked up a set of carbs for a honda 4 not realising they were 400, i am sure you will be able to sell them to recoup some cash
