I am having this exact same problem with my stock '71 340. Starts easy after 10-15 minutes but after 1/2 hour, will not start. Next day, started ok when cold but same problem when hot. I thought it was initially it was ignition. I swapped out the point distributor for electronic - no help. So either the fuel is vaporizing as the OP is suggesting or the opposite is happening, a slow leak of fuel past the seat causing a delayed flooding condition. To restart my car after 1/2 hour, I need to hold the pedal to the floor while cranking maximizing air into the carb. When it finally starts, it lopes to life like a flooded condition. I am convinced now, especially after reading this post that I am having a slow flooding condition causing the no start after 1/2 hour. Makes sense since the problem just started happening one day with no mods or changes.