I have done almost the exact same thing. I currently have Hx35W, 2150 psi calibrated 6.5 turbo injectors, 6.5 turbo IP with fuel screw turned 3/8" (maxed out), all on a 6.2 bored .020 over. 20 psi boost max with 5-12 cruising. ARP head studs (these are a must) and I'm still blowing gaskets.
Currently replacing head gaskets (both blew) with 6.5 standard Felpro. Ive used 6.2 Felpro standard, 6.2 Felpro .010" thicker, Felpro PermaSeal, Victor Reinz, and OEM GM head gaskets with and without Copper Coat. All of them have blown. If this next set blows, I'm going for solid copper gaskets.