Paged through and it didn't look like he used 400F pegs on that build.
I've got some insight, although for the record I didn't complete the conversion on the 550K I was working on as it went a different direction. I did my first 400F peg conversion on a 350F in 1997 and have done a bunch since, but only tried the 550 once.
But here is what I recall, and here are a few issues from the 350F conversion that might be relevant. First-- the pegs will fit and mount. On 350F conversions I usually cut down the brake side peg as it has that weird retainer part on the front. You may find some fitment issues depending on what kind of exhaust you run. My 550 had a Kerker 4-1, and it was going to be an issue that I would've had to work on. For the 350F conversion, I run the 400F brake lever on the right hand side, but it sticks out a little more since the 400F exhaust wrapped all four pipes to the right hand side to the collector. You could probably run it or fab it, but I'm not sure how your spring will mount or work, and your brake light switch may need some finangling. A 550F kickstarter will work on the 350F/400F peg conversion because it sticks out more, might have to do the same for a 550, or might not. On the LHS, for the 350F conversion I've had to grind down a little of the linkage so that it will function properly and not bind or be unable to complete the shift motion.
I think that's about it. I think that any of these problems can be overcome and the main thing is the pegs will bolt on (at least they did to a 75 550K frame). Good luck, and interested to see how this goes (and if it will do more to drive up the 400F front peg prices -- seems like when the 350F thing caught on 400F pegs got pricey. I'm sitting on 3-4 sets for future projects but when they run out I'll probably figure something else out... although there are still boneyards out there with them I suspect -- but not my local ones anymore!).
