If you are not certain of the gauge of your battery cables, you could start by replacing them with 0 or 00 gauge battery cables. A large-truck shop, Tractor Supply, or a welding supply can make them or might even have them made up. I'd bet that over half of the starting problems people have (cold or hot) is due to 12v cables being put in as replacements over the years.
The starter gets less efficient as it heats up, so a weak starter shows up when the engine is hot.
An outside problem is the engine might be tightening up when hot.
The generator/voltage-regulator and the battery are other possible culprits.
The first thing I did for the last three old trucks I bought was to buy battery cables. All three previous owners had different theories about why they were hard starting (or non starting, when hot). Problems solved with proper-sized cables (both battery cables).
Also, as soon as an old battery dies I spring for 6v Optima Red Top battery. My oldest is now 5 years old. It sits in the unheated truck all winter and starts right up every spring.
