1968 Dart sudden rear end noise.

to answer your questions on drop in, no muss, no fuss rear ends:

any a-body 7.25 from 65~76 will drop in, with the caveat that rear ends from 73~75 darts *must* be from drum brake cars, because they will be the correct bolt pattern.

a-body 8.25 rear ends from 73+ are big bolt pattern, they will drop in but you'll need to either run adapters or get new wheels to make that work.

a-body 8.75 rear ends are all small bolt pattern and drop right in.

for any of these, you may need to modify your driveshaft (shorten or change u-joint) and for the 8.25/875 you'll need the correct shock plates and u-bolts because the axle tube is bigger.

i'm in socal, i have rear ends and can help. PM me if you like.