Fred, I'm no expert on these, but I can tell you that the gasket is easily available from all the usual sources. The same gasket will have different part numbers, depending on make and model (1121939, 1318011, 663474 to name a few I found on Lightning Jar). Roberts Motor Parts sells them for $5.00.
I am in the process of putting a 3.73 into my Pilothouse truck. The 3.73 pumpkin looks to be in great shape, with little or no backlash. It has a different pinion flange than my truck, so hopefully the flange from my truck be a swap. Both my 4.11 truck differential and the 3.73 car differential take 10 spline axles, so that looks good.
Good luck with your swap!