Great story!!..,As far as the overheating….Your Avanti sat for quite awhile, so I think I’d start by flushing out the radiator/engine block … and also install a new 160’ thermostat….
Many Avantis tend to run warm in the summer, but if the coolant is boiling, there probably is a restriction somewhere in the cooling system… Best wishes.. Your grandpa would be proud of you!!
PS… if your ‘84 has factory electric radiator fans, several ‘84’s had the electric fans changed out for engine driven mechanical viscous fans, as the electrics did not cool down the engine as well.