[HennyM]

Let's assume (and you know what that means!) you've already done a compression test, checked plugs, ign wires, firing order, and a good fuel supply, you have an HEI dist & 12 volts to it, and there are no vacumm leaks. Check the pickup coil in the distributor. An ohmeter across the two leads should read 750 to 1200 ohms ? (or something like that) and should not change when you wiggle the wires. Sometimes the copper inside the little wires breaks and the insulation remains intact. This causes backfiring whenever the vacumm advance moves the pickup assy.
You can also reach in & give the wires a little tug, if the wires are bad they will break right off. It sounds as if you may have more than one problem. Double check for vacumm leaks; brake booster?, pcv hose burned?, Good Luck! H.