My guess is gearing or clutch slippage.
A 350 is not a big block. It's a mouse motor. It will spin on wet pavement...
A built 350 should spin the tires on dry pavement no problem. Especially in a pickup. Heck, my 216 will spin em on dusty or wet pavement without the bed wood and strips and my AWD Volvo with 325/354 will spin all four tires w/ traction control off. A first gear scratch w 300hp even at the crank should just be a matter of dumping the clutch.
