Andy
If you haven't, look at the v/8 forum I posted a link too. Overheating is a long standing flathead issue and there are some guys over there that has a lot of info on that subject. I am sure it will help you a lot to understand about the cooling of a flathead. They are like no other engine as far as cooling is concerned.
Larry

 

If it backfires when you let off the accelerator, it's a classic sign of the spark being too far retarded. You can do a quick timing adjustment, but at some point you should also make sure your points are set properly too, and then re-time the spark subsequently.

You said your radiator is OK, but are you sure? Is it original? Those passages in the core are pretty small and can get gummed up over time. A garage or professional radiator shop can do a flush for you pretty easy, but it'll cost you $50-$75 or so. If money is an issue, you can do a investigative flush yourself. There's a drain valve on the block and one at the bottom of the radiator. Open them up and drain your coolant into buckets for proper disposal. Close the valves and refill your system from at the radiator with water. Keep your hose handy. Start the engine cold, open up the valves, take off your radiator cap. As the truck runs, the water pump will circulate the coolant out and you will replace it with the hose water. Use a common gun nozzle to regulate the hose flow. After a little while verything should pee out clear, and with very good exit pressure (especially once the thermostat opens, if it's still in there).

Low pressure would indicate a coolant circulation problem somewhere, either core or head/block.