PA POW put Old school Air Ride on the 521 today.
Gabriel HiJackers Air Shocks 49346
Monroe MA802's were no longer available anywhere on the planet that I could find.
Note: I had to remove the rubbers and meatal bushings from the eye rings and use new rubber bushings to get them to fit on the upper mount. The 49346 hijackers cleared much better than my MA750 monroes, they rubbed on the axle housing, until one of them blew up, at least I think that's what happened.
You can use a MA750 but you will have to modify the lower mount to keep it from rubbing on the rear axel housing and popping the bag.
(Um, a friend told me... because of course I am too smart to do that the hard way)