I don't know what your truck had in it from the factory, sounds like a lot has been changed. I am assuming the rear gears were swapped. Any idea what the truck had for engine/tranny from the factory? Straight six and 3 speed had either 4.10 or 3.73, iirc. There was also a powerglide behind the 283, but I don't know what they had. The heavy half had 4.10 gears. If something had high gears, it was 3.07.
Sounds like you have a posi, provided both wheels turn the same direction. If so, you need to either buy gear lube additive for the posi or lube with the additive in it.
Best way to get the gasket is to count the bolts on the cover, should be either 10 or 12. I avoid the modern chain parts store, do most of my business at a mom & pop store. Yes, they charge a bit more, but the service is worth it.
