Don't worry about cutting those studs off - infact, its better you do if its stiff that try to turn them and remove them.
When I
removed my heater matrix, the bottom stud was stuck fast too. Because I kept turning it, it just ate up the plastic on the reverse - which meant a new stud wouldn't screw in either.
You'll see it better in that photo, a big spanner will take them off, its a tight bolt thread on the outside and a wide screw into the plastic.
I ended up using no more nails on mine to glue it back into the airbox to stop it from spinning. But even if you don't bother, the fresh air box is held in pretty solid just from the vents etc. around it!
