My brother has been killing his LLY and its still kicking! I can vouch for it as I don't know anyone that drives a truck as hard as he does. 12.22s and just going and going... 275k miles!
He runs big uS and a decent amount of timing and 38lbs of boost. We made 15 more Hp and 60 more ft lbs on the bottom end with timing alone, but backed it down some.
No 2 engines are alike, there is no true "safe"... Increasing horsepower decreases the life of the engine and grand no matter who or what tunes it. I have never failed a LLY or lb7 that I know of, but anything can happen at any time.
Personally, I take trucks to 650rwhp and generally they seen to play nice right there. We float the valves at around 60lbs of boost on most applications so tone them down to 57lbs.
Again, no 2 trucks are alike... But maybe this post will help someone in the future a little.