Which of these two 904's should I use?

the 68 should be the larger torque converter register hub which is what you need for any 68 or newer engine. Also has a different spline count on the input shaft so no swapping older converters in them. I'm pretty sure all V8 904 cases are the same. The /6 case used a single wrap band and didn't have the reinforcement ribs on the outside of the case.