Hi, I live in the UK and am in the middle of doing this conversion myself, I'm using the 280 calipers and the flat mounting plate. My solution is to have custom disc's made up that mean I don't need to machine anything apart from the centre hole of the bracket.
To me it seems a lot easier to have disc's made than bugger about with all the machining and welding , Don't you think ?.
The new disc's will be made of the strongest steel available and heat treated, Cost wise they will cost about £230 all in which isn't to bad when you think my old AP disc's for my pulsar where over £400 a pair.....
Rob