Biba, to my knowledge, your assumptions are all correct. You can easily make the pan gasket. I put a little bit of silicone sealant on the axles tubes whenever I reassemble. Unfortunately, I only have the pinion socket for the later 2L style diffs, but surely someone will loan the old style to you. If not, I'd bet you could make it easily enough with your skills. Unless there is reason to believe the diff was the reason the car sat for 20 years. I'd just seal it up, fill it with oil and drive it. That's what I did with my '56 after it sat for more than 20 years and it is fine. The diffs in these cars are much stronger than they have to be!