Did you retract the pistons all the way flush with the surrounding area? You should be able to put the pads on the rotor then fit the caliper over the whole deal. If the caliper is too narrow because of the pistons, you didn't retract the pistons enough. If the pads hang on some other part of the caliper then the pads are too thick.

I use a c-clamp against an old pad to retract the pistons. I turn the c-clamp screw until the pad bottoms on the caliper. Make sure the piston seal collapses evenly so you don't pinch the seal while retracting the piston.

Sixto
93 300SD