I think what I would do is install the rear end and see if the rubber line will reach the splitter block. It being on the drivers side tube will be better than on the hump of the center housing. Same with the brakes, I think what you have will work, and since you have small bolt pattern, there is no concern about a mismatch. Now, for the driveshaft, if it’s a 52” driveshaft, that is a really common size, and once you get the rear end in, you can confirm that is the size you need. I wouldn’t buy one until you do. Maybe it’s a Dart thing, because the Dart has a 111” wheelbase, and Plymouth cars are 108”, and the difference is all in the rear floorpan. I probably have 3-4 52” driveshafts here, which is the common size for a 318/904/7 1/4” drivetrain. Again, you have the parts to install the rear end, I would get it in there before spending any money on parts until you confirm that you need them. It may take a little longer, but it saves from buying parts you may otherwise not need.