Hi Kawboy,
I have done this conversion and managed to keep the bike so I can swap both ways.
I got my measurements by bolting on the rear fender to the back of the plate that crosses the seat rails. Then offer the rear tail light and licence plate bracket to the rear fender to work out where the new mounts have to go on the top of the rails. then offer up the A2 seat (mounts to same points) and rear tail piece to see how tall the rear bracket mounts need to be.
I wanted to be able to use either arrangement so I had to flare out the metal on the sides of the rear light bracket to go over the B2 rear seat/luggage mounting point.
I also had to leave some of the rubber bumpers off the underside of the A2 seat because the B2 front seat mounts occupy the same space.
I have some photos on photobucket
s1195.photobucket.com/user/zed_thirteen/...to%20A2%20Conversion
s1195.photobucket.com/user/zed_thirteen/...%20B2%20seat%20rails
Hope they help
Regards
David (UK)