There is not a really good safe boost its more like safe HP which i would say around 500
Wouldn't you say it comes more down to cylinder pressure?
and cylinder pressure increases with power (assuming injection timing is reasonable)
I tend to disagree on tune-only trucks. I just don't see blown head gaskets on them unless they're outright abused. I've seen more cracked pistons on tune-only trucks than I have blown head gaskets. Like WAY more.
maybe I'm just lucky, but I run big tunes in all my 6.4's and typically floor it to pass at least 10 times a day.
I've got buddies who, in my opinion, abuse their trucks... guys who couldn't keep heads on a studded 6.0... they'll crack pistons and/or roach transmissions, but not a blown head gasket between any of them on a 6.4
YMMV