If the truck had a bigger vac line plugged off, chances are the engine is from a F-4 or larger. They had a large port vac line going to a valve that bolted to the firewall ( 2 nut inserts vertically on the firewall near the center ) and a metal line went down to the booster underneath the drivers side. I know those brackets are not stock. I've ripped apart quite a few of these trucks and have NEVER seen anything like those. They're not for a horn as the stock horn is in front of the radiator and the optional long trumpets were bolted inside the hood sides.