I had one of the heaters you describe [the same "generic" dealer ones were common up to 59] in a 57 pickup and it did OK, but no heater will really warm up a panel, there's just too much space ... what I've done in the past is hang a curtin across behind the seats, let the cargo area stay cold, but back in the day there were aftermarket under seat auxiliary heaters some put in panels, burbs, and station wagons, which I plan on doing in my current project 57
Bill
