OK, maybe you're right. I've owned only CB350f frames, but both 350 and 400 engines.
Currently have a 350 with a 400 engine, with 350 pegs, and a 350 with a 350 engine but with 400 pegs.
I've always notices a clear difference in length, the 400 having a shorter kickstarter. But who knows if mine were original to start with...
Good tip on the 550 kickstarter though. Unrelated to this discussion, it might actually help me out on the 350 with 400 pegs.