Reading the "5.0 Cutlass", and the fact that its and 83, chances are that its a 307, 4 barrel with 5A heads. As for the gas issue, i tend to agree with the guys. My suggestion would be to look for non-firing cylinders, check out your Cat and check the Q-jet for flooding. My guess would be that the car has a weak/non firing cylinder, the fumes are mixing with the water due to the new cat, the plug becomes fouled due to the weak cylinder(Also helping the Egg smell) and you get a strong fuel water.