Honestly to go as far as the seal I'd go ahead and get new bearings pressed on your axles while you're there, they are 40yrs. old and will dry out and lock up eventually, my '67 did ~2yrs. ago and my '70 did back in 1988, once they lock you can't even roll the car. The bearings are like $20 each and a shop usually charges 1/2 hour labor to press the old ones off/new ones on ~$30-$45, call a couple muffler/ brake shops around you. If you have to do it later you'll have to tear it down to the same level as you do to do just the seals.
Here's a few pics of when I did mine, I rented the axle puller (in pic, looks like a slide hammer) from the local parts store free rental when I returned it. Lisle makes the seal puller sold at most parts stores, worth the investment, hooks and pulls them easily. Figure out what axle you have, get the bearings, seals, gaskets and fresh fluid, do it all at the same time and you won't have to think about it for another 20 years. You'll have to rebleed your brakes as trying to move the brake backing plate to the side will probably kink the line.
Good news is you don't have to remove the brake shoes/springs, leave it all connected and remove it as one piece once you disconnect the brake line, let it hang by the emergency brake cable or support it. That's about the easiest way I found for reassembly. There are 2 paper gaskets, one on the axle flange and one inside the backing plate.
If you don't have a shop manual for your car this is a good time to get one from any mustang parts supplier (NPD, MU, etc.) they're <$50 and worth it for the pics and torque specs. I keep all my notes in mine for casting numbers, serial numbers, part #'s and dates things were replaced like rear axle bearings.