http://www.stovebolt.com/techtips/bolt_removal.htmlTry propane torch heat.
Try penetrating oil and tapping/punching with a hammer on a screwdriver in the head of the screw.
Try to punch the screw from below.
Can you get a vise-grip on the head?
My WAG regarding why the "outside" screws are harder to remove: that's the wetter side of the road?
