GL-5 is supposed to be harder on bronze/yellow metals. GL-4 is supposed to be safe. GL-1 does not have the extreme pressure additives, and does not protect as well as the more modern oils.
Some oils have a GL-4/GL-5 rating, and are supposed to be safe for yellow metals as well as the latest stuff.
I have been rebuilding Ford Toploaders for 30 years, and have never had an issue with oils or yellow metals. What seems to work the best is plain, GL-4 or GL-5 NON SYNTHETIC gear oil. To be safe, choose GL-4 or one with the dual rating.
There are specialty suppliers that cater to guys with older cars. PennGrade makes a GL-4 90W non synthetic that works well in transmissions. It's only $8.99/qt. Not sure if they make heavier grades. There are reviews at the link.
https://www.summitracing.com/parts/bpo-77296