On both of my 64s (a II and a IV): from the center of the 12th fret to the centerline of the pickup, 10 7/8" (277mm).
...As far as a 'modern' Thunderbird placement, I'm both ignorant and apathetic, don't know and don't care. "Sweet spot" is determined by the characteristics/design of a pickup. About the only similarities with original and modern pickups is 2 magnets and 2 coils, differences are alnico vs ceramic, short vs tall, thin wire vs fat, number of winds and metal vs plastic cases. All of those things make the pickups very different sounding. Not saying one design is "better" than the other, just different. You can guess which one is cheaper to make.
On my '77 it's around 10 5/8" 12th fret to centerline. Those are REALLY different design pickups. I wouldn't worry about where the pickguard holes seem to be. Critical distance is the one relating to the neck.