Can I install an 8 3/4" axle assembly into an A-body without shortening the housing? I heard all I need to do is relocate the spring perches 1/2" inboard. Can anyone confirm that? Car is a 69 Dart.

Yeah all you need to do is move the perches in 1/2." You'll probably need new wheels because the offset needed will be a lot different since the housing is a lot wider than an A body.

I just did it, I installed a 66 B body 8 3/4 into a 71 demon.

Did it, just pushed the springs wthout moving perches no problem was easy.

Keep in mind that the 68-70 B body axle is the widest of the B body axles until 1971. The Demon, Duster and Dart Sport have rear wheel arches up higher on the bodyline, allowing more tire clearance than a standard sedan/hardtop Dart or Valiant. Comparing axle fitment on a Demon VS a Dart isnt completely fair. There is a spec chart of axle widths on this site. Check it out.

Used new springs, bushings, ect no problem with just pushing springs to fit see no issues.

I recently read a article (maybe at Allpar) about the design of the Duster. The designers really stuck their necks out on the car...truly risked their careers.

One thing I found interesting was that the designers planned for the rear axle to be 2" wider than what was produced.