Tried to take off the right peg,
I was right to think the bolt was keeping the peg mount in place, but they're not movable.
There's a stub that keeps the mount where it's supposed to be.
I was even thinking about switching around the spring in the peg seat to gain a cm, but it's not possible.
I really hope I can find an aftermarket brand that fits

wonder how tall people dealt with this back then, they probably just bought the 750 instead.
he also swapped the pegs from left to right.
This is on my '78 550 though.
If the peg mount is the same as Kenzo's then it looks like that would actually put the pegs on a lower spot since you'd have to mount upside down to swap them.
Sadly the cb400 has a different mount, so it wouldn't work.
Sitting more upright is actually increasing the risk of backpain and I therefor wouldn't recommend higher handlebars.
Sorry but I disagree, I've tried riding a klr for a weekend and I had no backpain whatsoever.
I probably didn't express myself very well tough, my backpain is caused by hunching over too much and I get pain just a couple of inches under my shoulderblades,
if I were to lean forward and reach the handlebar with a straight spine I probably wouldn't have any problem,
but if I try to keep that position I'm holding the handles with my fingertips.
If you are riding solo you might try moving your feet back to the passenger pegs. It completely changes your body geometry and I find it quite comfortable.
I guess the cb400 has a different geometry,
because if I did that I'd be resting my chest on the tank, riding superman style

As for the windshield... I don't know I never had much problem with the wind.
The real problem is that when I wear my riding pants my legs get so bent they become too tight and they cut the bloodflow behind my knees,
and also the aforementioned hunching problem.