I have always just made 1mm of shim with cut milk jug strips, and fit a regular SHA O-ring in back of the SHA.
The SHA doesn't always directly fit on sachs frames either. sometimes it can fit, depending on the model and nuances of the individual setup. They usually need a little extra spacer under the intake. I would probably get the arbeo with the air adjust screw. The SHA your biggest clearance issue is against the frame on the side of the carb, that screw sticks out in that direction. Also people have had to remove or cut down even the idle screw on the side to get it to snug up against the side of the frame.
Also it definitely does not fit under the sidecovers unless you chop out a lil space
still, it's a better carb and more easily and cheaply tunable. I'm not saying don't do it, but be aaware it's rarely just a bolt-on easy swap