The one original 6v battery would have been under the passenger floorboard.
The other 6v battery might have made it easier to jump start (due to poor original battery/cables/maintenance).
You should have no problem starting an original-location 6v engine if the truck has the correct-size battery cable and ground straps/cables (1, 0, or 00 gauge - bigger is better), and all the connection points are clean & shiny (and protected with an antioxidant copper-impregnated "grease").
