A lot of the 79 F150's were actual support trucks used at Indy. This can be verified with a History 999 from Ford. They were discussed on ProjectBronco.com and FullSizeBronco.com They apparently came with either a 460 or a 302. The 460 being the most common engine. When they were sold at the dealer, they came with the "Indy Pace Truck" decal for the door not installed. It was up to the purchaser to install them or not.
I own four 78/79 Broncos and a 78 F250 SuperCab so if you have any other questions, let me know.