Okay I found out the no start problem. I could see the carb was spraying good in the carb, so I went back to the electrical side of things and went through each component. I found that the wires connecting to the distributor side of the coil were loose. I tightened them up and it started right up.

I brought the timing in better and adjusted the carb since it was dry fouling the plugs and causing black smoke in the exhaust.

The car runs great now and starts perfectly.

So going back to my original problem. The starter works fine until the engine gets up to normal operating temp. Then it will barely turn the engine. If I wait a few hours to let it cool down below 100 degrees, it will turn over quickly and start normally.

So, next weekend I'm going to replace the new starter with an old one that worked fine before the engine was rebuilt and re-test. I had different headers on the old engine, so it will be interesting to see if the issue is truly with the starter or that I need a strong heat shield.