[hatzie]

The original 1513321 sender is what folks call a Nail Head sender. It's obsolete and the common replacements don't have the same thermistor resistance curve.
The gauge will not point to the same location with the replacement sender that it did with the original but it should still sorta work. This kind of thing drives the Corvette restorers nuts. Letric Limited has made an exact 1513321 replacement for restorers but it'll cost much more than a parts store unit. If you still have the original 1513321 sender I'd hang onto it and maybe sell it on evilbay or just re-install it.

60 seconds is not long enough to actually overheat but 4-5 minutes is... barely... I made the assumption that the coolant isn't boiling. That's why I suggested the infrared thermometer. Take several readings anywhere on the head block or intake coolant crossover keeping away from the exhaust ports. You could point it directly at the exposed sender threads or the threaded plug in the RH cylinder head where the sender would go if you installed that head on the LH side. Boy that last sentence has a bad flow to it...
If the coolant isn't boiling I'd look over the sender wire for damaged areas that are grounding to the engine once the wire warms up and the jacket is more pliable. A piece of expanding sleeve slid over the wire would stop one of these type of shorts.