Those are the cases I use, the one in the Falcon has easy 3000 passes on it with a 31 spline Mark Williams axles and matching spool. I've pushed as much as 500 HP through it on a regular basis with slicks and high stall converter that hits hard. Running about 450 HP right now in the car. The one in the Mustang has about 2000 passes on it. You can break one, but I'd think you'd be in the 600+ HP area, 650 or better more likely. Last one I saw spit out the pinion was a friends 6.60 Camaro and that rear had a bazillion passes on it.
When I build one, I deburr the case, especially around the rear support bearing. Use Timkin bearings and races, ARP studs on the caps and a solid spacer on the pinion. A good parts source is Randy's Ring and Pinion. I think its their Yukon kits, all good parts. Summit also has some decent kits with the good bearings. Add a quality gear set - Moser, Strange, Richmond, etc. Sorry, not Summit or Jegs. Never had an issue.
Note that new Strange iron cases are not very expensive and way stronger than anything factory, so if you are really going to hammer on it that is an option.
