What spark plug should I use in 1966 Pontiac 389?

You can't definitively say which spark plug is correct for a 1966 Pontiac 389 without knowing the specific engine configuration (e.g., 2-barrel, 4-barrel carburetor, HO, etc.). The spark plug requirements varied slightly depending on these factors.

To find the correct spark plug, you need to do one of the following:

1. Check your owner's manual: This is the best source of information. It will specify the exact plug type recommended by Pontiac.

2. Check the engine's air cleaner: Sometimes the recommended spark plug type is stamped or stickered on the air cleaner housing.

3. Look for a tag on the engine: There might be a tag somewhere on the engine block or valve cover indicating the correct spark plug type.

4. Consult a Pontiac parts catalog: A reproduction parts catalog (available online or from classic car parts suppliers) will list the correct spark plug for your specific engine code. You may need your engine's VIN or casting number to determine the correct specifications.

5. Use a cross-reference guide: Online resources and automotive parts stores can provide cross-reference guides to translate older spark plug numbers to modern equivalents.

Once you have the correct information, you'll likely see a specification similar to this (but with specific numbers):

`AC Delco [Number]` or `Champion [Number]` (or another brand)

Without this specific information, any recommendation I give could be wrong and potentially damage your engine. Don't guess; find the correct information before buying spark plugs.