I just looked up the parts, for some reason they don't list it as an add on accessory. I looked up the parts separately I don't know why you couldn't do it this way, but it wont come with instructions. Heater,68225550AA Cord, 68219818AA Bolt to hold it in block, 6501373. Ouch prices are crazy List price Heater $95, Cord $102, Bolt $4.75. I looked up the parts using my trucks VIN (it came with factory block heater) should be the same for yours. I will be on vacation but will do some more checking when I get back.
I want to figure this out for my customers too. Makes no sense it doesn't come as an add on mopar accessory. Im thinking an older 5.7 kit would work, maybe the cord is shorter?