The main problem is not the head gasket itself, but the open-deck design that can allow movement of the liners. The liner can shift just a minuscule amount, which is enough to allow combustion gasses to blow out the gasket. Under the stock design, the liners are only secured in place at the bottom. That's the reason for the o-ringed liners. You machine a ring in the top of the liner to accept wire, which sits just proud of the liner and will bite into the gasket when the head is torqued down. Also, most recommend increasing head torque from the factory spec (~70-75ft lbs hot instead of factory 65ft/lb). A more extreme solution to the head gasket issues (used in the bonneville spider making 400+hp) is to have a monosleve liner made (one big liner instead of 4 individual ones). That's a very expensive solution (~$5k) unless you own a CNC machine and have the skills to design it yourself.Vintre & Nealric,
Nealric, I'll look into some of the items you mentioned in your build. Is there an after-market head gasket for the Spider designed to hold more boost?
That being said, there's a user on Alfabb with a lot of turbo experience- Jim K.- who has recommended an Italian-made head gasket for boosted cars, but I'm afraid I can't remember the name of the company and I can't vouch personally.
EDIT: found it. The company is called Nava: http://www.navalineasport.it/eg/guarnizioni.htm