joedude
assuming there is a load tested battery (that passed the test). all cables and connections are good and connect to clean bare metal not a painted surface, the battery terminals are removed & cleaned to shiny bare lead and/or brass, there is no/minimal voltage drop when cranking between the battery and the starter, state of engine tune is good (compression test, spark plugs clean and gapped, plug wires in good shape with no bulges or soft spots detected along their length and rubber boots at terminals are good, distributor cap & rotor clean and terminals good inside and out, points gapped and clean and a check with white business card through contacts will not leave a dirt trail on the card, condensor good, coil good with clean tower wire connection and no heat discoloration on the outside or carbon tracking on the bakelite insulation, ballast resistor good and connections are clean, nice blue spark given on a test for spark-both hot and cold engine, valve adjustment within specs, valve train checked for wear etc that could lead to variation of valve lash, fuel pump pressure and output tested fine-both hot and cold engine, fuel filter good, air filter clean, a check for vacuum leaks is good, heat riser operational and not sticking, exhaust system not squashed or restricted), etc.
when you say it starts right up I assume that is from a cold start up. then when hot it cranks slower and doesn't wanna start. when you say cranks slower, do you mean like a nearly dead battery kinda slow or do you mean slower because it isn't also trying to start at the same time for some reason, it's just cranking a dead engine? trying to start means it will crank faster because it is in the process of starting. maybe try a cold crank over with the coil wire grounded for comparison. have you done a voltage check at the battery and at the starter when it is hot? starter draw test when it is giving trouble?
another reason for a slower hot cranking would be a heat sunk starter, a starter that is dragging when it is hot or possibly a poor connection in the starter/solenoid switch compounded when the hot starter is calling for more amps. do you have a foot starter switch or an key or push button operated electric solenoid style? here is a link to the hot starter issue that may explain some stuff better
https://www.knowyourparts.com/techni...hot-heat-soak/
another reason for not wanting to start after it has been running already is because of boiling gas in the float bowl when you shut it off hot. sometimes an insulating gasket between the carb and manifold will fix this issue as it acts like a heat sink/insulator between the hot intake manifold and the carb base. remember the inline 6 has the intake and exhaust manifolds bolted to each other with a heat riser butterfly valve to warm the intake on cold starts/days. the engine heat plus the exhaust manifold heat are warming the intake manifold right under the carb so it is common for the truck to smell like gas after a hot shut down because the gas is evaporating in the smoking hot carb. basically the carb absorbs engine heat after shut down so the gas in the bowl boils and ends up in the intake manifold and evaporates-you can likely smell it when you get close to the truck or lift the hood after it has sat for a few minutes. the bowl ends up nearly empty so the fuel pump has to refill the carb. that takes a few revs so the starter cranks the engine while it is not trying to start due to no fuel. it makes it seem like the starter is dragging but if compared to a cold engine, that is also not trying to start, the rpm of the starter may be the same or close. that is why I suggest to ground the coil wire and crank it when it's cold for comparison. the other thing that goes along with this is that the fuel in the lines near the engine also gets hot and can pressure up due to expansion until the pressure overcomes the needle and seat in the carb. more fuel to evaporate and also more fuel to pump in order to fill the voids caused by evaporation.
https://www.hagerty.com/media/mainte...ake%20manifold.
https://axleaddict.com/auto-repair/W...-How-to-Fix-It
voltage drop testing-remember this is a 12v system so the allowable differences can be different than a 6v system. this will tell you if the connections and cables are conducting like they should be. if using the old style braided ground cable ensure it is clean for the entire length, no corrosion, and there is a good ground cable from the body to the engine/frame. I think the old stock cables grounded to the cab on some trucks if i remember correctly. task force units anyway.
https://www.aa1car.com/library/volta...ts%20or%20less.
https://www.youtube.com/watch?v=U0OtjG4bwJg
hope that helped somebody.