Unless someone has messed with them before, those screws should be a #2 Phillips.
They can be a PITA, though. The screw heads tend to fill with dirt, just because of where they are.
I try to keep my trucks pretty clean. I remove those plates a couple of times a year to clean out the crud the somehow works it way underneath them. Holy crap! Where DOES all that come from? Leaves, grass, loose change, dog treats, MUD!
Add getting into the truck when it's wet outside, and any water will collect under the sill plates creating the perfect conditions for those screws to want to weld themselves to the body.
Because the heads of the screws are recessed in the plates, they suffer the same conditions as what is happening underneath.
Fill up with dirt, add some H2o, let it dry, & the heads end up being filled with a substance that approaches the consistency of cement.
Soak the heads with a release agent. 'Moove-it', 'WD-40', or any other rust eater. A couple of doses, letting it sit for a while between. Throw enough at 'em so that it leaches down to the threads.
Use a dental pick, awl, or a large needle to dig the crud outa the screw heads.
They should remove pretty easy now. If not, get the screwdriver into the head, & give it a couple of taps with a hammer.
Give it another dose of the rust killer, wait, then try again.
After all this, even if you get the driver into the screw head, be prepared for a screw or two to break, or for the head to get stripped.
Time to break out the drill!
Use a bit that's smaller than the head, but larger than the shaft. (That, uuuhhh, sounded a bit weird!) Try not to drill right through the plastic of the plate. There's an internal shoulder in there which is what the screw uses to hold down the plate.
Same idea as drilling out rivets. Once the heads of the screws have been drilled off, remove the plate.
With a pair of vise-grips, start to slowly twist the remainder of the screw back & forth to loosen it & get that POS outa there.
These things have caused me many a headache over the years. For a job that seems to be so simple, they can be such a pain.
Compared to some of the other vehicles I work on, these trucks aren't that old. I've spent a whole day to remove 4 plates on a vehicle from the '30's, '40's or '50's!