Hi Mark,
Here a few suggestions of companies I haven't seen much of (if at all) from the time period and whose branding I think would look great on a box truck:
Admiral TV's/appliances, Tide (to go w/your soap factory), Quaker State (motor oil), JELL-O, Pepsi-Cola, Sealtest, VanCamp's, Motorola, Kool-Aid, A&P, ALKA-SELTZER, Champion (spark plugs), 7up, Zenith.