Didn't you say that someone had converted it to IRS? Did the weld the trailing arm lugs in the 66 chassis or swap it to a later pan? If they swapped it to a later pan the # stamped on the chassis won't be the original '66 VIN and that might be the problem.
Did you get a GA title with it? I've heard several folks bringing a non-titled GA car into TN and had no problems, but I personnaly know of two cases that it turned into a nightmare and they were never able to title them.
The problem really isn't with TN, it's with the GA DOT. You don't need a title to get a tag once the car is more than 15 years old, so why pay $18 for the title transfer when you're holding the original signed title? The next time it changes hands it doesn't have a title and GA WILL NOT issue one. That's ok as long the car stays in GA (or AL), but almost every other state requires a title regardless of age.