OK, things are making more sense today. Turns out that the Spitfire caliper is not exactly the same. The mounting lugs are about 1/4″ closer to the caliper body than they were on the Jensen caliper. So, what was actually hanging up was the edge of the rotor.
Not to be dissuaded, I decided to disassemble the old caliper. After struggling with getting the pistons removed (compressed air works great to remove one, but then I had to figure out a way to seal the side I removed so that I could remove the other one) I took a good look at the inside of the old caliper, and it’s basically more dirty than anything else. So, I took some emery cloth and shined things up, and the inside of the caliper looks great now. The pistons look almost as good, and they all slide together nice and smooth. Thinking maybe what I will do tomorrow is disassemble the Spitfire caliper and swipe the seals and maybe the pistons to rebuild my old caliper myself, since I went this far.