[andyleonard]

The standard improvements on an 88 3/4 ton with a tired small block are new plug wires, headers and a cold air hose from the grill to the air cleaner. Almost for sure you are only getting fresh air up from the exhaust manifold, as the thermostatic air flap is broken closed. You probably have misfire from rotten wires. Plugged cat is a maybe, but if you have big tires and a big right foot, you'll never get much over 12/13 mpg with that setup.