building 8 3/4 489 section

Solid pinion spacer for sure. My thoughts are basically if you have "the must go faster bug" then over build it now. I would go with a forged or billet, bigger than 7290, yoke. A spool would be ok for the highway and think about 35 splines, I would back brace now and straighten with new ends if the finances are there. This way no matter what you do in front of the axle you'll never have to worry about that failure not getting you home, almost like insurance.