5th gear issue

[Ivan141]

I suspect only opening the gearbox will give you a definitive answer.
Every gear has to drive the output shaft, but 5th gear is the one closest to the output bearing (reverse is next in line).
So it might have to do with the driven gear being close to the output bearing and your output bearing being shot,
but it could just as well be a damaged gearset causing the noise.
I'd say give reverse a go and compare it to first gear (similar reduction), but whatever comes out the box will, as you say, have to be opened anyway.
And if I had a 40 year old gearbox open I'd try to have some syncro's replaced while I'm at it.. 2nd gear syncro is usually shot.