Just guessing- - - -once you have access to the truck, pictures will help- - - -that "egg-shaped" cover is probably a governor. Big rigs with 2 speed axles need to be protected from over-revving in the lower gears. I'm going to speculate that the engine might be a 270 or maybe even a 302. There were a huge number of military surplus 270's available dirt cheap after WW II, and a lot of trucks that were manufactured with smaller engines got retro-fitted with 270's during the 1950's and 60's. 302's came along in the 1950's as the biggest of the small and medium duty truck inline sixes. Unfortunately, hotrodders burned through the supply of surplus engines, and survivors are scarce and E$$$$pensive these days.
Jerry
