one guy was rebulidng a J-10 and said that he had used hot rod heater and updated the heating system in the
J-10. The J10 looked awsome. You can find hot rod heaters on google and they're expensive.
here is a NOS from ebay: http://www.ebay.com/itm/NOS-S-550368...item2c6027b749
the real question is, why are you replacing the heater. if you looking to add some heat you could keep your OEM heating and install a secondary heater and gain some extra heat. check out the mojave heater.
I have a YJ Tub and hardtop running half doors and soft upper doors and even went as far as buying a used YJ heating system.Replaced the blower fan motor with a bigger blower fan motor and use the CJ's fan wiring to turn the fans 3 speeds up to the highest setting and it is quite warm.