Overheating issues on a 383 Polara

I doubt it's a head gasket as the engine has only 69,000 miles on it and I'm pretty sure the heads have never even been off. Also the oil is still a nice light honey color, barely any combustion deposits and certainly no "chocolate shake" in there.

One thing that might possibly be the culprit is an odd block heater (coolant heater really) that was "spliced" into the lower radiator hose. I have never seen one like this so I have no idea how it works. Time to go look up my old Mopar Action tech articles on cooling, haha!

EDIT: Just thought also, could overly retarded ignition timing make it heat up? The engine had kind of a stumble when you'd step on the gas and wasn't quite as strong as I expected a 383 4-bbl to be, even in a 4500 lb. C-body. Come to think of it I sure hope the timing set isn't starting to wave goodbye...