That's not necessarily a bad purchase, depending on how much the junk yard is asking for it and how complete it is.
A-body housings are expensive. If the price from the yard is decent, you can have that '73 D100 housing shortened to A-body specs and the perches moved. Depending on where you're at and who you know this may even be cheaper than buying an A-body 8 3/4 housing. If it's a 5x4.5" bolt pattern axle you can just have the stock axles shortened, and all of the brake parts can be used too.