I have a 50 cad that is hard to start when hot.  I have a new 6 volt battery and a newly rebuilt starter.  I do not believe it is a vapor lock.  Any help will be appreciated.
I remember back when I had a 1952 Cadillac, which I wish I still owned. I traded it in 1962. I put many pleasurable miles on that car.

Well to answer your question, I had the exact same problem on mine. It would start perfect when cold put not well when hot, which I thought was the result of too large of an engine for 6 volts. The 1953 Cadillac came out with 12 volts if I remember correctly. What I did was buy a special 12 volt battery that had a built in solenoid on top of the battery whereas you would hook just the starter on the 12 volt outlet and it would start on 12 volts, and everything else on the car would still use only 6 volts. What a difference that made , car would start perfect on 12 volts, used it this way for about a year and a half before I got rid of it, didn't effect the 6 volt starter as far as I could tell.
Good luck with your caddy.