Most calipers have accumulated a good bit of rust and debris where the pads ride. This should be thoroughly cleaned with a wire brush. The debris and rust needs to be removed to allow the pads to install, however, of greater importance, is that the pads are allowed to slide on this surface. As the pads wear, the pad must move toward the rotor and any rust/dirt/debris will impede the progress of the pad.

I clean the cast iron surfaces of the caliper thoroughly and then add a touch of grease to those surfaces to stop the rust for awhile. The pads always slide right in.

Avoids the need to get your grinder out and go ballistic on the pad edges.