What causes the bolts in the head to shear anyway??? Is it some nupty who had tried to tighten (or remove) them for some reason or do they just 'go'???
I have recently had my Omega serviced and my mechanic mate told me that there is a slight blow from the manifold when cold, but disappears when warm. He is sure that it is the gasket from the manifold to front pipe. I have ordered the new gasket and associated studs and nuts for him to do the job next week.
I have heard that the head has to come off to extract offending studs. Bit crap though this sort of stuff to fail on quite newish motors! 
Dan
Sounds like you're in the same boat... sorry to say!
It's a common problem on the 2ltr omegas, maybe the V6's too?? I hope that i never need to do a repair job like that on a V6!
Anyway My 2.0 W plate sounded exactly as you described (noisy when cold but quiet when warm). My case turned out to be more than sheared studs and a shagged gasket; I also had a cracked manifold, which defiantly needs a GM replacement! About £100 notes from memory, not trade as i was a forum virgin at that time.
From memory I recall having x2 sheared studs to deal with, one of which was easy and the other was a right biatchh because it was one of the lower rear studs!! I removed the first one by drilling a little pilot hole inside the stud with a smallish battery powered drill. Keeping steady and taking my time. Once the pilot hole was drilled I used an extractor drill bit to carefully remove the first stud. )
The second stud being in a biatch location ruled out the battery powered drill as it was far too big for the small space available. So to get around this used my dad Ryobi ‘high speed, dentist style’ hand drill, complete with a ‘dentist style’ flexi-hose attachment. This enabled me to get right into the action and drill a small but accurate hole inside the stud. This worked a treat and again using the extractor drill bit I managed to remove the b****rd stud! )
I think on 2ltr Omegas the average driveway mechanic with the correct tools should be able to do this. It's not a difficult job in my book, more of a time consuming one. If you rush it you'll only end up making things worse. I think i did the studs plus rocker cover gasket in an afternoon. TD - you'll be ok with this, piece of cake!

Here is a pic of the tool i used to do the job...
