As for rarity, it's a no-brainer that OEM bars are getting scarce. But then again Addco currently lists A-body rear bars for all years, and has been making bars for these cars since they were new. I'm not sure how to tell if the bar you bought was OEM or an old aftermarket bar. I am also not sure if one bar is superior to another.
Most suspension vendors sell either Addco or other bars. They are and have always been a popular accessory. One word of caution, ALWAYS install a front bar first, never a rear bar by itself.
As for end links, get out the tape measure, you can get a set of universal links and shorten as needed. Bar should be horizontal with car on ground.