New to the Y-Block, so take it easy on a rookie!

Just purchased a '55 Mercury Montclair with, I assume, a 292.  I've found several resources for factory specs which reference the Champion 870 spark plug.  According to their reference material, the 870 is a 14mm plug, but the car actually has 18mm plugs in it (Autolite AP46).  Could this mean this engine has different heads than '55 factory versions?  Are there others that fit?  I haven't looked for casting numbers yet--I was hoping some of you could solve the mystery for me.  Maybe the mystery is just me!

The car failed emissions, which I half expected.  I'm going to do a basic tune-up ... points, condenser, coil, plugs, timing.  Even though the plugs look OK, the state smog czar says it's rich.  I thought about going to a hotter plug, and try to lean it out a little.  Any advice?