Can I switch my 350 Chevy engine to a 6.2 diesel?

The 6.2 was a dog (had an 82) and won't even compare to a modestly rebuilt 350.
Stick with a gas eng, maybe a 454, and you'll save a bundle and get better performance.
You could also install a Cummins but for what thats going to cost, you'd be better off trading it in on a better used truck.