"That threaded torque tube retainer cap is also suppose to have two O ring washers
."two metal washers (one on each side of the 0-ring)
That O-ring was a squared-edges rubber seal in the kits that I have received from reproduction parts vendors.
No leaks after many years.
Clean the torque-tube until shiny where that seat sets/seats on the torque-tube
Put a little grease on the mating surface/area.
If that area is not pitted, you might not get a leak there.Chevs of the 40sClassic Parts (Chevy Duty)Jim Carter Truck Parts