The Ford 9" is stronger than an 8.75" because there are three bearings supporting the pinion. One on each side of the pinion gear itself and the third out at the yoke end.
The reason Ford uses 3 bearings is because the pinion is mounted so low it absolutely has to have that rear support bearing to live. That does increase it's strength over one without a rear bearing but to say it's stronger is a very debatable statement since the pinion shaft diameter is so small on a 9". Remember, it's even smaller than a 741 case 8-3/4. There are several Ford 9" cases also. Even more an 8-3/4 Mopar therefore you have to get the right one to have a stout case that don't flex.
