I have a ground cable directly to the engine block. As this is an old car (1955 lots of rusty joints) I have every part, fenders etc. ground attachment places cleaned, copper based grease in between, and measured for resistance.
The starter is underneath the engine, the starter solenoid is mounted on the side of the fender. Even when I tested with the hood opened, and let it sit for 10 min. it wouldn't start.
I just did some tread reading on this topic. I did find 4 things.
1) Carter carburetor has heat starting problem
2) plug inside the carb tends to leak ( do not know which one)
3) heat leak from the intake to the carburetor. I did floor the pedal when starting, with no result.
4) timing to much off. (on this point, I checked timing when the engine was just warmed up and hot, no difference. Mechanical advance in the dif works as higher rpm will advance more). specification is 4 degr advance (1955 fuel) modern fuel should have a little more advance I think (run 95 octane).
Do you know more about leaking fuel into the intake, how to check and what to do about it.
There are some pictures of the carburetor and engine bay in this link