first time rear end swap

The previous trans was probably a 904. What trans is in your car now? If it's a 904 your good to go. I'd change the rubber line now if I were you. Did you check the bolt size for the front of the spring to see that it takes the same in the front hanger? I would also change the axle end seals before putting it all together. It sounds like your good to go besides those few minor things. Is that the original gear oil in the axle? To swap the disc brakes you will need the prop valve and disc/drum master. I'd stay 1" and under on the master bore.