Without ready access to both the pipes and a 400 I'd guess they are probably too far apart to make sense.
Obviously, anything can be made to fit but if you have to modify so much it starts to cost more than it's saving over scratch built.
Those newer four valve heads have the exhaust ports close together and central to the cylinders because the spark plugs are centered in the head, as opposed to the SOHC motors that have the plugs facing forwards with the exhaust ports effectively splayed out to accommodate this head architecture.
It's not clear if the newer pipes are double skinned either, which just increases the difficulty of cutting and rewelding.
[ Guests cannot view attachments ]
The above image seemed quote impressive to me as a one off modern take on those original curved pipes. I know it's a 550 but just shows what can be done. I've no idea of the source or cost of the above, but there are some pretty good pipe bending skills around to make it worth researching I think.
It really does set a bike off so well to have a good looking pipe installed.
