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The plug by the vacuum cannister should be the number 8 plug.
What is also important is what plug wire does the rotor align to for the proper firing sequence. If you know where the number 1 cylinder is at top dead center or compression stroke, then the relationship to this rotor position and the distributor tower where the access to the points and dwell are should be the number 1 spark plug wire, and going clockwise, the #8 plug wire would then be above the vacuum advance cannister.
mine is an HEI ignition. shouldn't the number 1 wire be on the terminal just to the right (clockwise) of the "protrusion" on the cap for connecting the module, tach, etc.???? if so, wouldn't that put the number 4 wire directly by (behind) the vacuum canister???
Determine your #1 plug position by the rotor position pointing on the compression stroke (finger on the spark plug hole method). Where it points is YOUR #1. It can be any position on the cap ....it's only in the "usual" place for convenient placement of the vacuum advance. It can be in ANY position. From your #1, go clockwise in rotation....doesn't matter if HEI or not.