a paint scraper might be able to fit in there and push the piston back. that is the problem, right, the piston is out , keeping you from putting the pad in. once you get some space a big screw driver (flat) well help.

next time, a small c-clamp attached to one side while pushing the piston in on the other, keeps the other side from coming out.

it is a good idea to push the pistons all the way in before removing the old pads.