62-65 A Bodies vs. 1968 A- interchange drivelines?

You will have to find some 64-66 slant 6 motor mounts, but other than that the motor and transmission will bolt right in. The rear end will bolt right in also, and you already know you may have to adjust the driveshaft length. The best way to get the transmission shift linkage to work would be to find a complete steering column from a 66 A body that was an auto on the column and get all the linkage that goes along with it. Your kick down linkage from the 68 should work fine.