XS400 Trippe Tree swaps?

Just because the steering bearings are the same for 1 model and another model doesnt mean it will fit.Some models use a longer steering stem than others and this wont be reflected in the bearing kits. Just because the bearings are differnt sizes, doesn't mean they can't be made to fit also.

The steering bearings are sized like this (XS 400 top bearing used for an example) = 25(inner hole dia/stem dia) X 48(outer dia/frame hole) X 13(bearing height). Your post list the XS 400 bearings, here are the XS 650 bearings = 25 X 48 X 15.2 and 30 X 48 X 15 (SSY500).

If the steering stems are the same length (I beleive they are, but you need to check to be sure), you would use the XS 400 bearing kit to install the XS 650 steering stem into the 400 frame. The stem diameter is the same for the 400 and 650(as shown in the 1st measurements in the bearing sizes), the difference is the outer sizes of the bearings and bearing heights and the size of the mounts in the frame.
DG

The XS 650 stem length is 225mm (the later 35mm forks - top of stem to bottom of bottom bearing). I don't have a later frame to measure.
The top triple tree is 23mm thick at the stem and there are 2 X 7mm locknuts between the bearing and the top triple tree. If you subtract the top tree and nuts thickness (total 37mm) from the overall length of the stem ,it gives you 188mm between the top of the top bearing and the bottom of the bottom bearing. Measure your stuff(stem,top clamp thickness and locknuts) and compare it to this info.(or put your 400 stem, locknuts, and top tree together and measure the distance between, bare, with out any bearings on it)
I tried to measure the early (1976) 34mm XS 650 stem and the measurements seem to be about 5mm longer (230 instead of 225) than the later 35mm measurements. It was hard to get an accurate measurement because the bike is together and not easy to get to, so it may actually be the same size as the later forks.
DG