The Curtis heater kit is fairly expensive but if you do a search on ebay for cab heaters you can find a variety for a lot less money. the only issue would be finding the fitting that goes into the thermostat housing to supply coolant to the heater. The return to the lower radiator hose can be easily made.
I actually have 2 heaters in my cab for those days when the wind is cranking and it's well below zero.