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If you are sure your HG is blown, then there really only are a few things to point a finger at. Being boosted, and running that much pressure, first thing I'd say is detonation. Its usually the first thing that goes with knock, because the HG is the weakest link when it comes to the extremly high cylinder pressure associated with knock. Most say that 2.5" is good for around 300 CHP, which at 18 psi, you are over, but not by a whole lot, either way, thats not your issue(although, if you have too much exhaust reversion back into the cylinder due to backpressure, it can contaminate your intake charge and possibly cause it to detonate also) As for the torque rating on the ARP's, they told me directly since we have a aluminum head, that the recomended rating is around 65 ft lbs. I personally put a bit more on mine, around 76 ft lbs, and run 15 psi so far without any failure. I know a lot of guys run much higher torque on the studs, and it seems to work, but i just took the route and suggested ratings. I even asked ARP about running lots of boost on it, and they said it will be just fine. The main reasoning is that the aluminum head needs room to expand with heat, and I would think that the more boost/power you run, the more the head will expand, and need a "looser" torque rating on the studs. Its up to you what you want to do, i have not had any issues, and others who run 100 ftlbs haven't had an issue, so take it for what its worth.
Next issue is making sure your head sealing surface is perfect, and the block sealing surface is perfect. Other than these few things, you should never blow the MLS headgasket.
I'd be more looking into detonation really. I can get my motor to ping for a second at a 17 psi spike on the S16G. I run tons of fuel and cold intake temps, and it still does it. If you are running all stock ignition timing, and at a minimum of 93 octane fuel, at 18 psi, i'd say you have a good chance that your motor is knocking. If you are not sure what it sounds like on our motors, take a listen to this sound clip. Listen carefully at around the 19 second mark or so. You will hear that rattle sound..
http://www.tridiamondpower.com/car_media/Jasondet.mp3
You can also hear this sound inside the car, with windows up(radio off of course), and it sounds like it coming from the passenger side area of the dash. 
13.5 @108 MPH-2.2 60ft(stupid FWD!)
S16G @ 18 PSI/FMIC/Running on MegaSquirt II (Now with sequential
fuel injection)
My webpage: http://eclipsed4evr.home.comcast.net
-1998 Mitsubishi Eclipse RS-T- "Toy"
-1992 Plymouth Laser Turbo AWD(SOLD)
-2000 Honda CR-V(daily)
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