I'm in the middle of a rephase project with my '78 XS650. Been "in the middle of it" for several years, unfortunately. Health problems have gotten in the way, but hopefully I'm working through the worst of them, and now all that stands in the way of completing the project is finding the time and the money.
I was attracted to a 277* rephase for a few reasons. First, I just really like the sound of 270* twins like Ducatis and Moto Guzzis. And of course I found the fact that the motor will vibrate a lot less quite appealing. Plus, my project will have a 750cc kit, and I'm thinking that, as a 360* twin it might vibrate even more than a 650cc, so all the more reason to rephase the motor.
I had Webcam (
http://www.webcamshafts.com/) regrind my stock cam to their 59A regrind, and had them grind it for a 277* motor. It is my understanding that Megacycle will do this also. If I had it to do over again, I'd probably go with Megacycle because I think their cams have better performance characteristics than Webcam's. I've asked Mikesxs a couple of times if/when they'll be doing a 277 cam, and all Mike said was that it's under consideration. That was a while back, though.
I printed out pics and explanations that were located at Bertaut's website on how to modify the crank, and took these along with the crank to a friend of mine who is a machinist and who specializes in machine work on motorcycles. The mods were easy enough for him to do. I haven't welded up the crank pins, though -- yet. I too wonder why this is strictly necessary and also how one will separate the crank in the future to replace bearings or rods.
I corresponded with Pamco Pete back when he was first developing his ignition system and he told me that setting it up for rephase was do-able and that all I had to do was request one for 277* (for a minimal upcharge as I recall). Hopefully he still offers this, because his ignition system is the one I was planning on using. Can't modify a Boyer cuz it fires a waste spark and that won't work with a 277* motor. Some folks have modified their points plate for 277* but I have no interest in running points and will consider that option only as a final and last resort.
Right now, I'm backed up with other projects, and I don't know when I'll be getting back to my '78. I have most of the parts for it now, just need to finish doing a bit more tearing down and then I can get started on assembly. I'll let y'all know when this begins.