Sorry, late to the game.
I thought the change in the register diameter was done in 64/65. 66 should have been the later version.

For what its worth, as long as the flex plate fits over the crank flange, the larger diameter register flexplate still works on the small register flange. The 6 bolts will hold it fine. At least it did on the one I beat the crap out of for nearly 60,000 miles.