I have a 67 with factory Borg warner OD. I love it......but, to add that tranny (should you find one) is not simple. You must buy a wiring harness and relay to make it work. Plus a kick down bracket and switch to mount on your carb base. Last you must buy an OD cable that mounts under the dash where you can take it in and out of service. If you go this route you must place the gear shifter in reverse if you want the truck to be sitting there when you go back to it. With help from this web site I found a supplier for the cable as I needed one for my recent resoration.
Was not able to locate a kick down switch so I fixed the one I had.
In regards to your rear end. I assume it is a 4:11. You can go to a 3:55 etc, but you may run into issues with the taller ring gear fitting into the axle housing. Plus that 6 knocker will be very weak taking off from a stand still with a taller gear.
Lastly, if you go with a 700R4 you must fabricate a support bracket for the tail housing. That long tranny housing may push you for space due to the drive shaft bearing hanger. I understand the 200R4 comments. I do know that tranny is used in drag cars so it can be built to be strong. Good luck
