Looking at a few pictures of that diffy.
I am going to assume the case is aluminum (Common)
The threads are quite vulnerable and do wear out over time.,
Helicoil is a great way to repair bad threads.
As far as the "Gasket" goes.......If gaskets are not available.....use HIGH TEMP BLACK RTV.....OR THE RIGHT STUFF
The goo in a tube will give you good sealing properties and is easy to come by......
The right stuff is good....but a bit more $$$$$
On my OC12 the axle tube mounting threads on the drop box covers were sad to say the least.......I cleaned the threads well then inverted the covers in the mill and countersunk the holes so a flat head Allen bolt would thread in from the inside out.....
I added loctite to the threads to seal them well.
Now there are "Studs" to hold the axle tubes into the housing that will not pull out
Looks like you may have enough room inside the box to be able to thread bolts in from the inside out ?????/
Loctite these and snug up well.
Then you can use Nylock nuts and washers on the outside.
Add the Gooooo to the surface.....tighten those bad boys up and you should have an oil tight assembly that will be very solid....