Axle again...8 3/4"

What do you mean by back plate? I will assume backing plate for the brakes. The price sounds good if it is for sure a sure grip. If you are retaining the small bolt pattern you will need the brake parts of of a rear end with smbp. Likely your dart will have a 7.25 diff. If they are the 10" drums you can use all the parts from your existing rear diff. If the 8.75" rear end has backing plates (must be small bolt pattern ones....offset is different for big bolt pattern) you are good, if not I believe you can modify the 7.25" backing plates to fit (you will have to drill an extra hole for the bearing retainer.........I haven't done this but I have compared the parts and they should work).

Now if you want big bolt pattern you will have to redrill the axles and drums (and keep the smallbolt pattern brakes) or buy aftermarket axles and get the big bolt pattern brake assemblies from another car (the 8.25" stuff will work)

ps. Greetings from Alberta........