Sure grip 7 1/4 strenght?

Got a friend with a 375hp 318, his 67 cuda tore up the first 7 1/4. I think he said he launched gears through the case. His 2nd 7 1/4 he shredded the gears inside which was good so he just had to replace the gears. He is working on his next set of gears right now, I can't hear the howl, but he says its starting already.

If you aren't shocking the rear end you'll be find, but if you do what my friend's been doing... yeah short life.