In your climate, you can get away with an actual phosphate finish, but up North with the Spring and Fall heat and cold cycles, the phoshate finish will quickly turn to rust.
If you want the phosphate finish, I highly recommend dark gray phospaint by OEM paints. It has the correct sparkly finish and is very durable. This paint has a darker than orginal gray phosphate color, but the metallic in it looks great.
For what it is worth, I own several 69 Chevelles and the hood hinges came in either of two finishes, dark or black phosphate and black painted. I'm not sure what the normal coloring of the 70's is but it is my guess that it may be either of the two finishes depending on who manufactured the hood hinges.
Good luck.
Woj