Re: 1978 302 timing questions
Start with basics.
Set the engine at TDC (top dead center) on the compression stroke. You find TDC by setting the #1 cylinder on the compression stroke (both valves closed) at it's highest point in it's stroke. You will probably have to "rock" the crankshaft back-and-forth to find TDC.
You can fabricate a crude indicator with a coat hanger and place it in the spark plug hole to find the piston's highest stroke. When you have established TDC, mark the dampener. You might be surprised to find a groove or degree marks already there. If you do, fill it in with white paint.
Now install the distributor with the rotor pointing close to the #1 wire terminal on the distributor cap (mark the distributor body with a felt tip pen to make this task easier), but keep the distributor clamp bolt just snug, not tight, so you can rotate the distributor by hand.
Connect all wires and connect your timing light to the #1 cylinder. Turn the ignition on, but DO NOT TRY TO START THE ENGINE. With the ignition on, rotate the distributor counter-clockwise until you get a spark, indicted by a blip, on your timing light. Hold the distributor and tighten the hold down bolt just enough to hold the distributor in that position. Mark the distributor so you can find that position again.
This method is not ideal, but most engines run with the distributor advanced. You will develop a "feel" for how far to advance an engine, but start by turning the distributor just a few degrees counter clockwise, tighten the distributor bolt enough to hold the distributor in place with the engine running, but loose enough so that you can still turn the distributor by hand. You should be able to start the engine with the distributor in this position. Have someone help by starting the car while you rotate the distributor SLIGHTLY.
Once the engine fires, SLOWLY rotate the distributor until the engine idles smoothly. Mark that location on the distributor body and adjust from there by trial and error.
The best method is just to find the correct timing marker parts and install them. Then set the timing per factory specs, around 6-8 degrees as I recall, but CHECK for your particular engine.