Give the pan a thorough cleaning including submersion in a bath of warm water plus the Original Blue Dawn or Simple Green liquid soaps. Let the pan soak so the emulsifiers in the soap have time to work.
Blow dry the pan paying particular attention to all seams. Then wipe the pan with lacquer thinner, and apply two light brush coats of POR-15 Chassis Black.
POR-15 will fade from direct exposure to the ultra-violet spectra of sun light, but the oil pan does not get direct exposure. However, to assure gloss retention, scuff the POR-15 and overcoat it with a gloss black single stage polyurethane with a hardener. The combination of POR-15 and urethane is one of the hardest and most durable paint finishes on the planet.
An aside from finishing the oil pan is that, the original oil drain plug receptacle was upset into the oil pan like a hollow rivet. Ford did not seal, solder, braze or weld the drain plug receptacle into place. The Ford design oil drain was notorious for spinning when the plug was being removed and for dripping oil, so it became a common practice for shops to solder or braze the receptacle to the pan.
When done inside of the pan, the solder or brazing of the oil drain receptacle is not visible and a highly recommended practice.