7 1/4" to 8 3/4" rear end swap advice

Put the leaf springs on the rear end first. Then mount the front perches first, so it hinges up when you jack up the rear end. It will help keep the rear end located. The driveshaft for a 8 3/4”/904 combo in a Dart is 48 5/8”, but I would have the rear installed and under load and measure cup to cup to confirm that. And I would consider having your existing driveshaft shortened. Now, as mentioned above, there are 2 different A body 8 3/4” housings, a 66-69 and a 70 and later. The only difference is that the vent tube and line splitter block was moved from almost on the round section of the housing, down to on the drivers side tube. Depending on which housing you have, you may need a rubber line from a C body car, or a hydraulic extension for your existing line. I used an extension thst I got from a guy who runs a mobile hydraulic hose maker that makes lines for semi trucks, they’re trucks are usually well equipped with adapters of all kinds due to their roadside service nature.

So, you have everything pretty much covered, although I would consider using at least 10” drums up front, or going with those disc brakes you have, if the bolt pattern matches your new rear end. Otherwise, 2 spare tires may be needed.