I had the misfortune of having the front windscreen and rear window break at different times, and both times the insurance's recomended company made a pigs ear of the job. (Autocarglass/NL)
If i were you i would get yourself a pair of safety glasses (health and safety) and stand over them like a hawk whilst they do the work.
When my windscreen was done on my 156 they broke the scuttle panel, lost 4 out of ten of the clips that hold it in place, and failed to reseat the rubber on the scuttle correctly, allowing water ingress into the engine bay. they then told me the scuttle panel wasn't covered under the insurance......!
The rear window, was no better and there are still greasey handprints on the interior trim by the rear pillars that i cannot get rid off, and i was left to refit the trim myself as they forgot to do it! (I was in a hurry to get to work, so didn't check it properly at the time.)
I am now very guarded about how the work is done, but still believe the specialist glass companies are the way to go, as r11co says.
Just make sure you know what to watch for as far as scuttle panel removal and refit go, and don't be afraid to make any 'helpfull suggestions' if they haven't done an Alfa GT before.
Have a look for relevent threads on the subject, i found plenty for the 156.
Good luck with it..