I'll tell ya my personal experience.
I was trying to get a 1940 GMC windshield frame out.. it's the same as the '41 thru '46 Chevy windshield frames.
Anyway.. the bolts.. hell I think they were flat heads.. anyway.. they were stuck tighter than heck.. and heat was one of the first things (after gratuitus use of WD40 failed) I tried.
Now the glass was kind of foggy and cracked, so damaging it wasn't a consideration.. but I can report that hitting it with a propane torch for several minutes didn't break it any further.
I'd be most concerned about the laminate thats on most of that windshield glass.. but provided you don't hit the glass head on with the flame, you outta be in good shape.
I only managed to get two of those bolts out.. one from each side.. I ended up using a dremel to hack the heads off the remaining two, then used WD40 and a set of vice grips to back the bolt out afterwards.
