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For several days before you actually start the removal, soak the distributor opening in the manifold with a rust buster (PB Blaster is recommended). [Also soak the top bolts for the exhaust manifolds if you're removing the heads.]
First crank the engine around to where the timing mark is at Top Dead Center and the distributor rotor is pointing at the number one plug wire on the cap.
Note where the vacuum advance is pointing and where the number one wire is sitting. (Often the factory position for these are not what you have.)
When you remove the distributor the rotor will turn as the gear is at an angle. Note the amount it turns as it will need to be offset this much during assembly.
Now disconnect the battery.
Drain the coolant and be sure it is below the deck by removing the small screw in plugs at the bottom edge of the cooling jacket. If they are stuck - as they often are - then knock out a freeze plug. If there is coolant in the block it will run into the cylinder and down into the rings. While not a disaster, it just is another mess to screw with and I like to see the cylinder walls as they were when the engine was running, and not after I've cleaned up the coolant and wiped out any signs of problems.
Drain the radiator, remove the valve covers, carburetor, thermostat housing, distributor, rock arm assemblies and push rods (and keep track of which side that stuff came off and which push rod belongs to what valve), wiring that runs across the intake manifold.
Remove the temperature sending unit, and any heater hose connections, etc., that you want to install on another manifold (they're a lot easier to remove while the manifold is still bolted down).
Finally the intake manifold itself can come off. To loosen it use a pry bar or large screw driver at the center and lift each side loose. If you have a cast iron manifold remember that that sucker is heavy around 80 pounds so plan to struggle. I place a folded old blanket on top of the fender so I can set it down without denting the fender. If I'm lifting it out by myself, I actually stand inside the engine compartment.
You've got to clean the head gasket surfaces, block ends, valve cover gasket surface, and thermostat housing gasket surface.
Make sure the gasket fits OK at the bottom of the ports on the manifold and heads. This step should not to be skipped as some manifold-head combinations require a special gasket.
Dry, test installation is next to let you check for manifold alignment if your using a different manifold or if the old one had sealing problems. Make sure the manifold port faces line up parallel to the heads. Use the distributor to align the manifold front to rear especially if is a replacement manifold.
Re-assembly is next. Use carburetor/throttle body cleaner to get the gasket mating surfaces, especially the block end surfaces and the undersides of the end of the manifold, free of any oil so the gasket sealer will seal better. I use either VersaChem 99 or Permatex Ultra Blue 77 and build up a bead half again as thick as the dry fitting showed me that I need. I let it dry several minutes before putting the manifold on.
After the intake manifold is in place, install the distributor before the manifold bolts as the distributor determines the front to rear alignment for the intake manifold.
Most everything else is straight forward, just get things back where they came from.