I'm not so sure about a 66, but 64 and 65 A bodies had the 3.09 low gear, whether six or V8 powered. Great low gear for 3.23 gears; gives you a 9.98 overall low gear ratio. You'd need 3.73 rear gears to get the same overall low gear ratio with a 2.66 low gear. (And 4.56 rear gears to get that overall low gear ratio with the famous close ratio GM 4 speed with its 2.20 low gear.)
I know from experience that going from 3.09 low gear to 2.66 will absolutely kill a 340's off-the-line performance when running 3.23 gears.
Some early seventies 318's also used a 3.09 low gear non-overdrive 4 speed. (Not talking here about the overdrive 4 speed, which also had a 3.09 low gear.)
