Unfortunately 50oz bars don't usually attract much of a premium even the older ones.
As a guide I'd only be looking at slightly more than the premium on a modern 50oz bar (maybe an extra couple %)
However, if its got a great patina, and if you have the original 1975 receipts (or original bags/boxes etc) to go with them, then you might be able to get a bigger premium.
Can you post some pics? (do a search to find out how - its a bit harder than normal until you get over 10 posts)
And welcome.