Removing Blazer Rear Hatch Lifts

[buckkoo]

I'm going to replacing the jam-prone sliding lifts on my 72 Blazer with gas-pressured lifts from Strong Arm per http://67-72chevytrucks.com/vboard/s...d.php?t=220511

My difficulty so far, is in removing the 2 Phillips screws which secure the lift brackets on either side. After 44 years, they are not inclined to move. I've sprayed them with penetrating oil - no luck.

I've tried an impact screw driver. I'm guessing heat is out, since we're working with fiberglass.

Any ideas?