Being 50 years old has nothing to do with it. Third members still bolt up to the banjo housing. I may have missed the whole point, but axle housings are not made out of sheet metal. I would drain the grease and remove the leaking bolts and look at the threads on the stud as well as in the housing. If the threads in the housing are indeed stripped, you have two options. Pulling the third member and installing heli-coils. But that is an invitation for oil to past the threads. I would recommend reaming the holes in the carrier housing and then drilling and tapping the holes to the next larger size. Poke around with a magnet to pick up filings and install a magnetic drain plug.
I really doubt if the threads are stripped. If the threads are good, clean things up and apply a flexible sealant. I have been using Permatex No.3 for almost 60 years and it works. Thread sealants are for tapered pipe threads. You are dealing with straight threads.
If the gasket is leaking, a quick and permanent fix is loosing the carrier from the banjo housing to where you can pry it back about 1/8". Then wrap about 3 or 4 rounds of butchers twine around the whole thing.
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If it is the gasket, the guy probably has a point. It is difficult to get a good seal between the third member and the banjo housing with the old thin gaskets. That is why I use RTV and have been using it for 25 years or more. The same goes for OEM. However, once you have installed the unit and had oil in it, you will not be able to get it clean enough for RTV to work without removing the third member.
