Is my rear end strong enough

The 8 1/4 (actually 8 3/8) pinion diameter is 1.625. That's 1 5/8 for the decimal impaired. The GM 12 bolt pinion diameter is........wait for it.........1.625. I've seen 12 bolts take in excess if 1000HP. Course they had good stuff in them. I'll be damned If I would swap in some inferior phord crappola when what you have will work just fine.