I am doing this very conversion to my 84 1200. However I also installed the front forks, wheel/tire, brakes and fender from the 1500 too. So, my bike will have a 18in front and a 16in rear. I have the front finished and it fits great. I still have the rear to do.
I am doing this to a 1200 Aspencade that I have stripped naked. I wanted the better tire sizes so I could find good ADV type tires. I also wanted to get away from the Aspy air suspension and the 1500 donor does not use air in the front. For the rear I am using 2 no-air 1500 shocks.
BTW the 1200 looks good with a 1500 front end on it.