Does it look like the pads are fully seated against the piston and are the back of the pads agains the caliper body? If so then that is as far as you are going to get them. Sometimes the pads are just thick enough to make it really hard to slide them over the rotor. I would guess the pad maker went over tolerance.
Or if it is just a snug fit, you need to make sure the pads are fully compressed and perfectly straight as the slide over the rotor. It only takes a tiny bit of wrong angle to make it seem impossible.
If the pad is not seated flush then you need to get the piston down deeper, I like to use a big C clamp, something like a 10 incher should do it.
Sometimes the piston just isn't going to go deeper due to corrosion in the cylinder, in that case you will need to either rebuild the caliper, fairly easy if you have the cylinder hone and parts, or just buy rebuilds, often just as cheap as doing them your self.
With the age of your car, it is a fair bet you have some corrosion unless you maintained the brakes every couple years or so.
Good Luck,
Jim Henderson