Rebuilding calipers on an '03

Guys, Everything went well, disassembly, buying new seals, etc. But for the life of me I cant get the piston back in with the new seals.
There is no rust, corrosion, etc. The old seals were is good shape when I took them off.
Since I'm doing a rebuild with added supercharger, I thought, well why not make sure everything about the brakes is known good.
Now I cant get the piston back in using the old seals. I've tried a C clamp, but no go.
Any ideas?
Thanks

Well, both front and rear. All 4. As luck would have it, I started on the rears.
I made a simple rotation tool out of a socket and tried twisting as I applied pressure...no go!
I'll go at the fronts today, but any help would be appreciated.
The pistons are phenolic I think.

Beautiful job on your rebuild! Love the white mustang.

5L5,
Thanks for your help.
I finally got it. Gonna need some touch up paint!
The key was the initial alignment of the piston as it enters the bore and hits the inner seal. It must be perfectly centered/squared in the bore. Then the applied force used to seat the piston all the way must also be square into the bore.

The caliper itself doesn't afford any square surfaces for a c-clamp to sit on, so when using a C clamp, the seating force gets cattywampus and everything stops.

A nicely sized piece of wood (the end of a 2X1 I think) put on the piston square (with the caliper held by a bench vise) and then hammered gets the piston past the seal, which I imagine is being compressed a decent amount into its seat and against the circumference of the piston...which I suppose is absolutely required.

Thanks again!
-KK