‘67 barracuda heat soak and timing issues

You need a tachometer reading to go with the timing measurements. 5* is low unless its an original distributor in which case it might be correct.

The timing may or may not be related to the engine overheating. Need to see the rest of the advance curve. But yes that is very much worth investigating.

Overheating while stopped or moving slowly is just as likely due to the lack of fan shrould and possibly the distance to the fan from the radiator.
Looks likes like a passenger side water pump, so long snout.

Headers will make the engine bay and floor boards hotter, especially when not moving.