Yeah, there are two sizes. I have a 78 F100 without AC and it takes the smaller one. Not sure if this helps.

 

I didn't realize at the time that if would effect EVERYTHING. I want to get this truck on the road. The path of least resistance is to just fix the heat and leave the AC disconnected (could hook AC up someday?). I started taking it all apart today. I haven't got to the blower motor yet. The blower motor resistor heated up cherry red and damn near caught the truck on fire....melted the wire harness too. I'm really surprised the fuse didn't blow before that thing got so hot. I took pictures ...if I can figure out how to post them.
Also I'm in the market for a new blower motor resistor and the wire harness that connects to it. I think I may have tracked them down but I need to get back out to the truck tomorrow and confirm that they are they same.

 

When I fixed mine I found it easier to take the hood, right inner and outer fender off. Then its not too bad, you can take the whole box off and fix it on the bench. I took all the inner blend doors off, then cleaned and painted it. Works perfect now.

 

Here is the updated link to the North Country Specialties tutorial which includes changing the fan.
https://www.ncspecialties.com/shop/heater/index.htm

 

Thanks for that Brad.

Paul