5th ave, diplomat, gran fury, pre 83 lebaron, are all M's.

the shocks interchange with J's (mirada, 80-82 cordoba, 80-82 imperial) and F's (aspen/volare)

they have an eyelet on the top, bayonet on the bottom. I don't know if a late B or R has the same shock layout, since they also have the isoclamp system.

shock mounts were never designed to carry vehicle weight. Air shocks and coil-over helper shocks. and are a bad idea to try increase rear ride height and load carrying capacity. the right way to do it is to get better springs or overload springs. a new set of 5 leaf HD police spec springs should run less than $250 from ESPO. they added over 2" of rear height to my 5th ave over the stock sagging 4 leafers....conversely, over on dippy.org's forum, bill55az has found the front segment to be the same length as a dakota, and has used dakota rear springs with good success on his old M body wagon with some mods to the rear spring hanger.