Intermittent problem with Backfires that occurs after 40 minutes

[DeathProofCuda]

When the car is cool and will turn over, check cranking voltage at the coil.  If you are at less than 9.5 volts then resistance in the primary circuit it too high.

Also check out the attached tech bulletin on ignition systems from 1969.  Perfect for your original points car.

[Chryco Psycho]

Fuel & ign problems can often be confused
 
 Fuel side - lean which seems most obvious to me as you say it stop if you add more fuel , so fuel level & jetting are the first things to check , I find if you are running stock fuel pump increasing the fuel level slightly really helps on Eddy carbs , eddys are also notoriusly lean out of the box & float levels are never close .
 
 Exhaust leak is another possibilty this can change with throttle .

 Ign side , weak spring on the points causing floating but would not change with throttle , Weak coil usually breaks down under more load not less but still possible , not enough coil voltage  , poor Ign wires , wrong gap / heat range Cold / poor make , NGK is the worst they love to misfire &  foul
 
I always runmax timing at idle & limit advance curve , without causing pinging , typically the best is to set your timing near max RPM at idle then hold the rpm at around 3000 rpm & see how much timing you have to back off to get it to run its best , if you have an adjustable timing light   you should get 2 number around 16* & 38* , mechanical advance should get you close by itself , maybe add a few degrees of vacuum or just leave it disconnected .
 

[bdschnei]

May sound simple but...Check your grounds. Sounds like it could be a bad main ground that's building resistance as you drive until it gets to the point where it's impeding current flow to the coil. Since the starter requires even more current to operate it won't crank until the resistance in the ground circuit decreases by cooling off. You could temporarily route an additional ground cable directly from the battery to the engine to see if it makes a difference.

[DeathProofCuda]

The parts on these cars are inexpensive and easy to replace. In a worst case scenario where it can't be determined the cause, what are all the parts to blindly replace and in what order? I assume the voltage regulators (both dash and firewall),  ballast, coil, & points, first all at once to save time. Then the  distributor, alternator, fuel-pump, carb one at a time. I bet I missed some parts, what are they? Thanks for your help and any other suggestions would be greatly appreciated.

https://www.youtube.com/watch?v=3xjLtg3FLoA

Get example of why not to be a parts changer.  Owner replaced both batteries and the starter on this truck, when all he really needed to do was clean up the battery post connections.

[Rich G.]

My guess would be the starter getting hot is why it will crank when it cools off. I'd do a starter draw test and a volt drop on the battery cables to the starter for the crank issue.

[Brads70]

Might be more than one issue, cranking might be the green death inside the battery cables?
I had that one bite me once. How is your coil mounted sideways or up and down?