.
Pull the plugs.
Pull the valve covers.
Rotate the crank until the valves on the cylinder being tested are closed.
Use the leak-down tester to test the best cylinder first, to get an idea of what the air flow rate of a good cylinder is, then proceed to test the problem cylinders.
Listen for air flow in the crankcase via the holes where the air breather or PCV valve are inserted.
Listen for air flow in the intake manifold down the throat of the carburetor.
Listen for air flow in the exhaust system at the tail pipe outlets.
Look for bubbles in the coolant system at the radiator cap.
>A small amount of air flowing into the crankcase is normal, but an excessive air flow rate means bad rings or a holed piston.
>Air in the intake manifold means a tight intake guide, no lash, bad intake seat, bad intake valve, a bent intake valve, a factory valve in the process of the head breaking off the stem, a failed valve spring, retainer or lock.
>Air in the tail pipe means a tight exhaust guide, no lash, bad exhaust seat, bad exhaust valve, a bent exhaust valve, a factory valve in the process of the head breaking off the stem, a failed valve spring, retainer or lock.
>Air in the coolant system means a cracked casting (cylinder wall) or a blown head gasket.
> Air coming out of the spark plug hole of an adjacent cylinder means a blown head gasket.
A cracked casting is what I'd be worried about; since the engine has pop-up dome pistons & quench heads. The 351C engine block is known for cracked cylinder walls. But a bent exhaust valve, resulting from the valve kissing a piston dome, is also likely. Like Steve I'm hoping it's just the gasket.
In spite of Ford's advertising, the 351 Boss had a bit less than 10.5:1 static compression. That was pushing the limit. The factory spec for the thickness of the cylinder walls was 0.160 inch +/- 0.030 inch. The cylinder walls could be as thin as 0.130 inch (3.3 mm) off the show room floor. Not much thicker than a Japanese paper lantern. With a 4.030 over-bore that could be as thin as 0.115 inch (2.9 mm). Compression higher than 10.5:1 static or 8.0:1 dynamic turns the engine into an IED.