Like many others replying, I have also experienced this problem over the years. I attributed it to heat soak. If the motor turn over more slowly when you have the non-starting problem, it almost certainly is heat soak of the starter. The slower turnover may also accentuate a timing problem as already suggested. Especially likely if one has headers, although in your case you do say your motor is stock.
I reduced the problem by wrapping the starter with a heat shield. If you are a McGiver kind of guy you could probably make one out of sheet aluminum and some of those blankets that they sell for protecting surrounding areas when using a propane torch. I could maybe help you with some drawings if you cannot find one ready made. I got mine from JC Whitney in the 70’s!
You could also be getting some boiling of the fuel out of the bowls and/or vapor lock in the fuel line. A phenolic insulator under the carb and/or a heat shield that extends under the bowls could help. I have heard elsewhere that Jegs sells a 1/4 inch thick phenolic space that works with Holley carbs. Thicker is better, but hood clearance with the air cleaner top becomes a problem. The way the fuel line runs along the side of the block by the exhausts and up the front of the motor also exposes it to a lot of heat. Maybe an insulating sleeve over it in that area would help. When I get my new motor in I am going to run lines up the back of the motor along the firewall using "xtreme nitros" insulated fuel hoses. Will need some flexibility/slack to allow for engine rock in the motor mounts.
I also will be going to one of those smaller gear reduction starters as others have suggested. It will keep the starter away further away from my new headers that are also jet hot coated to reduce engine compartment heat.
Gene
PS: It just occurred to me when you say your motor is stock, does that mean a 2 barrel carb with the hotwater heated spacer under it? If so, that spacer might be contributing to a heat soak problem with the carb. Just a thought.