Fortunately there are enough of the older 6.0's out there so you might be lucky and find one that's had the bulletproof treatment already. What Vic said about the 7.3 is spot on. The 6.0's are hit and miss, where as the 7.3's are lots more hit.

Personally I'd find a Duramax with an Allison behind it and be good. No gamble there.