I have a mercruiser thunderbolt distributor that has a flat section on the side for the module. How can I tell if it for a six or eight cylinder engine? Any info would be helpful.

It will work for both you just have to have the right rotor and cap and make sure you run the right gear with your camshaft.

that would depend what rotor came with it,if it has the rotor still in tact count the gaps between the shiny parts