Nice looking bike, reminds me of the Triumph 750 Bonneville I had but that bike caught fire outside an Austin nightclub.
Those 400Fs were pretty bikes but yeah, factory frames in that era were flexible.When Kaz Yoshima rebuilt my carbs, they no longer had chokes, they were smoothbores, this was 1977 after all. I would lean the bike way over, and flood the bike, one kick, and it would start. I had it worked out perfectly. With everything I did to that bike, I knocked around 50 lbs. off of it, it was a very light CB400F (CB458cc actually, after new pistons, etc.). It was very easy to hit 120 mph on that motorcycle. The famous Honda wobble made it kind of scary though.