The largest one, or use whichever one matches the driveshaft you'll use. You will most likely have to shorten a stock driveshaft or have one custom built for use with the Tremec, I got a custom one for mine, so get whoever builds/modifies a drive shaft for you to include a yoke for the larger of the two u-joints. I always went with the largest joint that would fit. Don't race mine either but you never know when you'll grab some good traction while having all that torque applied. I consider the U-joints to be the weakest link in my drivetrain.
