A Body swap with E body 8 3/4 with 4link

I hate to be this guy but, you're in trouble with that rear end.. it's too wide, so to do it correctly it really needs to be shortened, the seats will need to be moved and you'll have to coin up for custom axles.. (Moser, Strange.. ) which may make you realize you want rear disc brakes, which may require a brake system upgrade. BTW, do you need e-brakes in your state? I see the original cables - which probably won't fit your Dart (I don't know for sure, just thinking "Murphy's law") even if you stay with the drum brakes.
You're going to hate this advice but, search for a correct size 8.75 - hell, put up a wanted ad on here.. and then join the E-Body forum and sell the one you have.