AC Delco part # AC 41-993 is the plugs used in the 5.7 and 6.2 Black Scorpion motors. In my years of working on motors stick with o.e. spark plugs for best results. I.e. GM use AC Delco, Ford use Autolite, Mopar use Champion, Honda, Toyota, Nissan use NGK or Denso, German cars use Bosch.
Search around there is a large price difference on the 41-933 plugs, some of the local auto parts stores are $13.00 per plug can find online as cheap as $5-6 per plug. Mercury uses a platinum plug so little more money than your standard copper plug.
http://ac-direct.amazonwebstore.com/ACDelco-41993/M/B000NWA6TU.htm