Intermittent backfire with loss of power, briefly. Under load and driving on flat.

Check dist cap for cracks or carbon tracking. Check coil for cracks around tower or spray water on coil, wires and cap and see if it makes a difference. Check carb for nuts being tight as they used to loosen up as well as float bowl. Take air cleaner off and see if top of carb will shake by hand with engine off if it does the float bowl is loose. Check for a cracked manifold, both of them. As well as a vacuum leak. Make sure wires are on correctly 1-5-3-6-2-4 firing order

Check your compression it's a good possibility that you have a burnt valve, short each cylinder out one at a time to see if the backfire or spitting goes away. It may be compression leaking through valve making it sound like a backfire. Could also be a head gasket as well.