I would offer a clarification. "Correct heater" isn't the best term to use since HJs didn't come from the factory with heaters. The Parts Book shows 2 heaters packages #100171 and #100238. The parts book shows these packages as being the heater unit, fittings, brackets, and switch. The heater unit in the later is from the earlier big cars. The basic unit in #100171 is smaller and resembles a common Arvin aftermarket unit of the time.
Since HJs didn't come with factory heaters, any reasonably installed after market heater could be considered as acceptable for judging.
The parts book section for #100171 shows L&R fresh air vent packages but it doesn't appear as if they are included. The section for #100238 doesn't mention the fresh air packages at all. Neither is going to be very effective without a right side fresh air vent to duct air to the heater.
I don't think finding the 2 heater units discussed would be difficult; the hard part would be finding the switch and switch bracket and the correct cables to operate the fresh air vents.
And a special note to Gordie: Allstates are supposed to have Allstate branded heaters. You would have to consult with Hewlett about whether the Sears store installed fresh air vents or just slapped on an off the shelf heater.