1971 Chevrolet Truck Engine fire

1971 350 V-8 During cold start, smoke came from hood. Carburetor area was on fire. Put out in less than 2 minutes. While cleaning area, I found a plug wire going to the distributor, which had a split in it. On the side of the cap was a burnt are which tells me this was the cause of the fire. The plug wire's are not very old ( about 3 yrs. Old). What could cause this? Sometimes on cold starts the engine would backfire. Your help would be greatly appreciated as I do not want this to happen again. The truck is garaged kept.