Hm,  I was thinking that shaving the existing NDS endcap down 6mm would have the effect of moving the rotor to a suitable place while reducing the OLD spacing to 142 at the same time.

Haven’t sketched this out so quite possibly I’ve missed something.  While it sounded like exact frame was a huge variable in whether it would or wouldn’t work from the MTBR thread, some people there were finding they had enough lateral adjustment on the caliper mount to fix the brake alignment (guessing this would not have been the case if they needed to find all 6mm!)

Rebuild is obviously an option and I even have a spare 350 rear knocking around which would only need endcaps although I’d definitely need spokes as the OE hub is SP and mine is traditional J-bend.  In fact, thinking about it, I have a set of suitable rims already so it might make more sense to leave the nice new wheels for the next bike and just get a front …

Longer axles were being used by the guys in the thread – typically they seemed to be buying 148 axles in the same fitting for their frames as otherwise they were barely getting thread engagement (understandable).  Some were flexing their frames, some claimed not to be.  Someone raised the question of ‘whats it doing to your pivots’ and someone else responded that with the shock removed they couldn’t feel any additional drag while the wider hub was in the bike than when the correct item was fitted.

On the whole I think I’ll probably just reckon on a wheel build to suit, but it would be interesting offering the boost wheel up to a few frames to see how close it was.  Based on the other thread, the answer (that I’d always assumed) of ‘it’ll be 6mm too big’ might not always be the case.