66 used a different kick down, didn't it? And thus maybe a different throttle rod? Just speculating again.
Yes, '66 used a different auto trans kick-down. Instead of being an add-on piece bolted to the intake, the accelerator linkage at the firewall was modified and a cable setup used.
The '65 and '66 MPC shows C5ZZ-9A702-A on the line for 260/289 2/B carb , 11.67" long from center of ball to rod end
The '65 and '66 MPC shows C5ZZ-9A702-C on the line for 289 4/B carb, but does not give a description or length
The '75 MPC shows C5ZZ-9A702-D for 65-67 260/289, all and 11.56" long
If you notice, there is a '68 small block version that is an inch longer, but carries a C8ZZ part number.
I've attached a snippet from the '75 MPC with all the versions and sizes.