Well, it has to turn to be able to fit the new pads. If yours won't turn, it might be frozen up. Do you have the special tool to turn the piston, or are you trying to do it with needle-nose pliers etc??
Here's a proper tool kit for many different types and styles of brake pistons, not too expensive:
I have one of the little cubes with a different pattern on each side, and it's worked for me in the past, but it's not that great of a tool. Pretty easy for it to slip.
These kits on Amazon (and plenty of them on eBay, too) have a nice big drive-handle and the correct adapter disc for your application. So a pretty solid tool that you can get a good turn on.
If that piston ain't wanting to go in with a tool that has more leverage, maybe it's rusted in there. You may need to get a rebuilt caliper.
Rockauto is still selling rebuilt calipers for the Gen I at reasonable prices:
RockAuto ships auto parts and body parts from over 300 manufacturers to customers' doors worldwide, all at warehouse prices. Easy to use parts catalog.
www.rockauto.com
I checked out the workshop manual, and it said there are 2 ways to remove the brake pistons; 1) pop it out by disconnecting the brake line and applying air pressure (like with an air nozzle that has a rubber tip), or 2) screw the piston counter-clockwise. If you can get it screwed-out of the cylinder, the seal kits for these are pretty cheap.
You might try pulling the caliper, stick it in a vise or other suitable clamping device, and use some heat from a torch on the body. Then see if you can get the piston out.
So is it hot enough for ya down in TX? It's been a terrible spring up here in the PNW (cold, rainy) and the weather is just starting to get nice. But we had a real Scorcher last week with temps over 90, then it cooled down into the 60's and rained! Just no figuring it out! 79 and sunny today and then it's back to rain on Sunday. Crazy!
G'luck with the repair............ed