I'm pretty sure 66 up had 6 leaf springs on both sides. I know a "67" 4 speed Formula S 273 and "68" auto Formula S 383 both had 6 leaves on both sides. I never had a 65 Formula S.
I just gave my buddy a pair of original 66 springs from a Formula S, 6 on the passenger, 5 on the driver. I’m not saying that’s gospel, just what I’ve seen through the years.