We've done a mess of 6.0L trucks and the sad reality is that there is no such thing as "bullet proof" when it comes to head gaskets. The closest you can come is pulling the entire engine, having the heads AND the block surfaced, heads pressure tested and magnafluxed and repairing as necessary, using studs, quality gaskets and o-ringing the heads. It would be best to do the coolers while it's down too.
Most people don't do the block or o-ringing, and I would say that 99% of the trucks we do don't come back...provided they had us surface the heads (no warranty without the headwork). It's that 1% that is the most aggravating though...trucks where we did do the head surfacing, did use new studs, did use quality gaskets, did the coolers and they still have a failure. We've had a few where we ended up having to pull the block and have it decked (rare, but occasionally necessary).
The simple fact is that installing head studs, machining head surfaces and upgrading coolers is not "re-engineering" a poor design. There are still only 4 studs per cylinder, the load is still not as evenly distrubuted as it should be, it's still a 6.0L...just with a few upgraded parts and higher tolerance machine work. It's better than stock, but expecting it to be as good as an engine with more evenly distributed clamping force (18 bolts per head like a 7.3L or Duramax) is asking for too much.
Don't get me wrong, I'm not trying to be "Mr. Negative"...it's just that we talk to so many 6.0L owners day in and day out and the expectation is that throwing some aftermarket parts at the engine will make everything "the way it should be". In reality, it makes them better, but still less than ideal. For the average 6.0L owners daily driver, with moderate modifictions and typical towing usage, most can expect a significantly more reliable truck with these modifications. For those on the more extreme end of the performance scale, the chance of a failure will always still be there...just hopefully prolonged.
