It's 5/8, unless it's been seriously abused.

I have one of pretty much the same vintage (within the first year of
Mark V construction - no access hole on the backside of the headstock,
green paint, Magna Engineering). I recently upgraded the quill/drive
system to the "newer" style, which was a few hundred $, but it got me
the new two bearing quill, as my old quill was sloppy after 50 years of
use, and I had never gotten around to doing the DIY two bearing upgrade
on it; AND it meant no more bleeping Gilmer belt (swapped the drive
parts to go to the Poly-V belt while I was at it). Them fancy new parts
swapped into the Olde Machine just fine. So there is hope if your quill
has, in fact, been abused.

--
Cats, coffee, chocolate...vices to live by