What are you running for a motor?
If you are running a V8 that I would go with the 700r4. Good low 1st gear, plus overdrive. You can have it built real nice and strong. You can adapt your stock t case, but that would be the perfect time for an upgrade. Isotel here did a 700r4 with a Dana 300, and built a mini truck / cruiser hybrid axle for the centered rear diff. Lots and lots of upgrades for the dana 300. Or you could spend the ching and go with an orion.
If you are running a stock motor I'd stay away from an auto. Sucks too much power out an already underpowered motor. I can't even think of anyone who makes a conversion kit for the f series motors. Up and Over innovations or downy made one at one point, but I don't know if they do anymore. If you are running a stock motor, I would either go with an orion, or with a marlin crawler.
Stay with the stock torque converter. Any lower and you will drive through your brakes.