Not sure I've ever read anyone doing it, but that's probably as much to do with so few of the originals left, as much as it not being possible.
I would at least try it. You really want disc brakes if you can swing it, but I understand that it would be nice to be original.
I'm not familiar with the innards of the 3-bolt vs the 6-bolt hubs, but isn't the internal mechanism basically the same?
The problem that everyone runs into with a disc conversion, is the same problem people run into when swapping out the original lockouts for aftermarket versions like Warn and MileMarker and such. You can't retain the inner axle (smaller) snap-ring in 95% of the cases.
It's the same whether you're swapping to discs and using your stock lockouts, or swapping to different lockouts. Just not made for each other.
I'm sure there have been diagrams showing the guts and inner workings of the lockout of a '66, but I don't have it handy. Do you happen to have a link, or have you studied them enough to know the differences (if any) of the old vs the newer locking mechanisms?
Paul