742 vs 489 Case

742 will work best, it is a stronger case.Even if you use a crush sleve eliminator, it is still stronger.
But back brace it.I blew a 489 apart (broke the caps off)within 10 passes ,so I went to a 742 which lasted for years. It was in a 3200 Duster, back half 14X32 tires.650 H.p. B.B. with Griner brake.
It packed the front wheels on every run, and ran 10:70's all day long.
The ticket is, run the street gears,not the pro gears.The street gears are softer, set them up tight. You will still need to check the ring and pinion every year, most likely after the 3rd year you will find chips off the ring gear. Even with the back brace.
The issue is, the car leaves, but the tires stay, this bends the housing, and pulls the ring away from the pinion.
30 spline will work, but 35 is better.
Hope this helps.