You may not actually blow the head gasket in a big puff of steam, but you can weaken the fire ring enough to cause a minor leak if you go past 10 PSI. The engine will probably still run fine and might not even have a coolant problem for a while but its possible to actually have the fire ring on the head gasket get deformed enough to allow combustion gasses to get past and cause erosion of the head and block mating surfaces.
When I tore down my engine for the overhaul I had this exact problem even though I had good compression (410 was the lowest) and good power. No coolant probems either, but there was slow and steady erosion happening in there without me knowing.Some have gone to 15 psi with stock 6.9 head bolts. You might just get away with it, but I'm most comfortable with telling people 10 PSI is the safe/reliable limit for a 6.9.
If my engine still had stock head bolts, I would probably enforce a 7-8 PSI limit, but tend to be more cautious and prefer reliability over power. Even 5 PSI will make a big difference in power over N/A.
I thought 12 psi was okay... I've hit 12psi only for a short period, but try to keep it under 9 psi just to be safe. BTW, I have a doors on my '86 parts truck, but shipping would be pretty pricey.
