Can't tell if the oil line is an old or new line...and same with oil pressure gauge.
If old, acquire a new one from FNH. Because of the small gauge line, and if rusted or thin you can predict that a crack or break may occur again.
Contact George Bradish for a black bezeled oil gauge if you are wanting to get the correct type of 80# guage on your your 600/800.
The fitting should snug just fine onto any of the Ford type of gauges.
But, you may want to try some teflon tape around the threads and see if that works.
I'm working on an 871 with 703 loader right now, and it is now apart and in pieces in the drive, and various shades of the old paint, primer, grease and blothed white from zinc phoshpate. Three layers of old paint on this rig!