how strong is the 8 3/4 #741 case?

I don't think a 741 differential is measurably weaker than the 742 or 489. Typically when an 8 3/4 breaks it's not because the pinion snaps. Usually the caps will blow out (driver side), the cross shafts will grenade, or the gear teeth will simply shear off.

I'm no expert so don't take my word for it, ask the doctor:

Pinion Comparison - Quality Body Shop Drivetrain