I had the same problem with my 1948 Plymouth, I change to new cables (2 gauge of course), Selonoid, Starter. I charged the battery full made sure it was working. After 4 years and 2 batteries I gave in and paid for a Optima 6V battery from NAPA Auto parts. Guess what I have no more problems. The car starts right away. Even after driving it for about 30 to 40 minutes Starts right up. Before when I drove it for that long it would not start. It wanted to but sounded tired. The power that Optima gives is a lot more than the regular 6V. Try it. I know some people hate them but works for me. That is all that matters. I also have a D24 Dodge, When I am not sure I take parts from it and to see if it works on the other car. This way I know what could be the problem. Helps me a lot,
Good luck, LFT