Garbage Bags and Duct Tape?
I suggested to a female cube-mate of mine a few years ago - who was complaining about the high cost of diapers for her infant daughter and how frequently they needed to be changed - that she use garbage bags with two holes poked in them for legs and copious amounts of duct tape. She didn't see the humor in my suggestion. Her husband thought it was a good idea though.
Seriously, with any sealing surface that you need to make sure doesn't leak, you need to do a couple of things. Make sure both surfaces are consistently flat and level (ie no dimples where the nuts tighten down on the sheetmetal covers). You need a gasket material that won't seep the fluid you are hoping to contain. Most fiber gaskets will seep 80w-90 gear fluid over time. So any gasket material you use must be impermiable to gear oil (like dead soft aluminum or copper). You can also use silicone, but older trucks weren't built to a higher standard that newer trucks were - their sealing surfaces don't mate as well as later ones do. Consequently, you need to make extra sure that the sealing surfaces are extra flat and mate well, as in my first point - if you plan to use silicone. Silicone works well in situations where there are tighter tolerances between mating surfaces, but performance drops off rapidly when you have looser tolerances.
Make sure any plug welds are in good shape - they have a tendency to leak, and may need to be rewelded. Be sure to replace the pinion and axle seals. Make sure both pinion and axle bearings are in good shape - if they have any slop at all, the seals WILL leak.
