Mike I don't see that one of your questions has been answered. You can not add overdrive to the AD transmission (nor any torque tube transmission). But you can add overdrive to a Saginaw 4 speed transmission with a bit of work and it makes for an interesting (however some think not all that practical) transmission. Given that the overdrive transmissions out there are over 50 years old now and all, the T5 (as others have mentioned) is a really good option. If you go with overdrive (or the T5), you will need to abandon the torque tube and stock differential. Good luck.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end