As long as the string grooves aren't too sharp and shaped decently, I think the risk of catastrophic failure on any vintage bow due to the use of FF strings is very low.
A lot of people advocate padding the loops with B50, but I doubt it's even necessary as long as they're thick enough so as to not start sawing into the limb.
It's pretty amazing how those old bows shoot with a decent string. I'd put my '64 Kodiak up against any modern recurve and come out just fine....and yes I own a pro-chrono and use it.
