From what I have discovered the 400 supplied to the UK (single front disk) shares "most parts" with the 250's apart from......
Frame, some differences around the engine mount points.
Engine...Totally different
Tank and side panels.....Again Totally different.
Carbs and airbox.....Totally different.
Rear Tyre is 140 as opposed to a 150 on the 400's
Crank ignition pickup has a different resistance when tested electrically.
As you say some parts (single front disc) and rear brakes, wheels and levers/controls are the same as other Suzuki bikes.
Try searching here for parts.........www.wemoto.com as they also provide dimensions of parts for comparison.
Pete