I can't figure out why it failed. You where not that hard on the throttle, and it should be able to spin to a higher RPM than 100 mph. Who made that prop shaft Chevy original, local drive shaft shop? (I am betting it wasn't Denny's). Also what size bearing was in it?
I didn't see any evidence of drive line vibration, so I am assuming the pinion angles are all set correctly.
Finally does the car have data loggers and full sensor package; such as drive shaft RPM?
I am disturbed that it let go like it did. At least you know that the drive shaft safety loops work.
A GM R3 cup with a 1310 in it I assume? I'm not sure all of our members are aware of the difference between the newer R3 that replaced the older Spicer 1310. Think of it as similar to the problem of going metric on our Novas with a SAE bolt in a metric threaded hole. Close but no cigar!