Did you force the piston all the way into the caliper? If not, the new pads won't fit. One way to do this is with a large "C" clamp. While you're squeezing the piston into the caliper, the brake fluid level will rise in the master cylinder reservoir, so you'll have to monitor that level.

Or, attach a hose to the caliper's bleeder valve, route to a suitable container and just barely 'crack' the bleeder to allow excess brake fluid to escape. You'll also find it takes less force to fully compress the piston back into the caliper.

Something else to keep in mind while working on these: it's very common for the caliper sliding pins to sieze up. If the caliper doesn't move freely back-and-forth on these pins, you'll have erratic braking action and the pads will wear unevenly.

If I recall, NAPA stocks the rubber boots and other hardware for these.

If you find your calipers are really Hammered, a set of rebuilt calipers is the best way to go. You can get them "loaded" with pads or "unloaded". Just slap 'em on and go!

HTH and G'luck.........ed