I, too, have read my way through all of the forum comments about which color gray to use. In the end, I went with what my local NAPA had on the shelf, which happens to be the Dura-Kote "Ford Gray" (cough, gasp). I like the color as it's very close to the shade of gray used by GM's Electro-Motive Division (EMD) on their diesel engines.
Brian
'51 Chevy 3604 Project '28 Chevy LO basket case '83 GMC Sierra 4x4