Does anybody out there have a recommendation for a 3 pt. post hole digger that my old B7500 can actually drive?
I don't think driveability will be a problem. My BIL has one on his Ford 1520, which is similar to your B, and it has plenty of power. Depending on tire size, lifting the bit high enough to clear the ground could be an issue, but you probably have enough adjustments to make it work. Be sure to check driveshaft length so you don't push it into the tranny, you may need to shorten it.
(I started typing this 3 hrs ago and got sidetracked, so 15 other people have probably responded by now.

)