Ive been running my conversion for some time now, ive done about 6000miles running 12 psi on stock gaskets and my baby is still going strong,
Please do not think this is safe though i think recomended on stock head gasket is around 6-7 psi we only run these boost levels due to the fact our actuator wont let us run any less and i have a spare engine wich is currently being built to a low comp spec.
Speak to turbo fit i believe they have succesfully run 5-6 of there na-t converted sups up to 473rwhp dont know what costs are involved in this though sorry.
one last thing if you do decide na-t keep some cash back, since completing my conversion ive had to shell out for new clutch, larger injectors, greddy e manage and a few other bits.