Hi RBrown,
Welcome to the web site! This is THE place for hobknobbing with vintage Chevy folks.
I don't know the exact answer to your question, but here is my take on buying the 600W oil.
Considering that the transmission, the u-joint, and the differential all use 600W, you are looking at roughly 1+ gallon to take care of all three.
Instead of paying a lot per quart, I opted to buy a 5 gallon bucket of it from a local industrial oil dealer. Granted, I have two vehicles so I use roughly 2 gallons per year. I think going this route is cheaper than buying the Mobil 600W by the quart.
Cheers, Dean