I feel unless your doing a Concours restoration or want the "Just left Detroit" look, it is pretty much what ever peels your banana as to the color blue you like. I personally like the dark corporate blue with the chrome (or aluminum) valve covers and air cleaners like you would see on the 390, 428, and Boss cars. Ford did have a part number for Corporate Blue engine paint (EP-2075-C) quart, and (D2AZ-19500-AB) 16oz spray, but are now discontinued. You'll also hear many different numbers given for everyone's favorite version of 1966/ Ford blue. There again personal preference. I've always used Plasti-Kote #224. Having been involved in MANY engine projects over the years for friends and myself, I found this color best matched what was on the engines we pulled. In fact this is what we used at the Ford dealer to paint shop built engines and to touch up Ford built engines going in customers cars. A friend of mine who restores Mustangs used the Plasti-kote on his personal 68 GT500 KR Shelby and it looked really great!