brake pads won't fit

Hey all,
This is seriously stressing me out. I'm trying to replace the brakes and the rotors on my 73 plymouth duster. Discs up front and drums in the rear. I've got the cylinder pushed back as far as it will go but I need like 2mm more space to get these pads on. Has anyone else had this problem? can I sand down the pads a little? The new ones look the same as the old ones.