Actually, 6303 just means crankshaft, it's the rest of the letters and numbers that determine exactly what it is. Unfortunately I can't tell you from the info. in the parts catalogue what type of crank you have, the stamping numbers aren't always shown.
E9SE 6090 D7A is the regular 89 - 93 SC head casting. The rest might be date codes or something.
'ESSEX' refers to the engine plant in southern Ontario near Windsor where the 3.8L motors are produced.
cheers
Ed Nicholson
SCCoO