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5.0L (1979-1995) MustangTechnical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.
I would say drive it around for a little while (30min or so) and then let it sit for a few days. After its sat for about 48h go back out and pop the hood. Pull out the oil dib stick and see if there is little "bubbles" or drops in your oil. If it looks like when veggie oil is in water you have coolant leaking into your oil which would explain the greyish smoke. If that is so I would take that engine back to whoever built it for you and smack them for buying ****ty gaskets or forgeting some bolts, LOL~!