Here we go again, trying to do long distance analysis of a cooling problem. without knowing there really IS a cooling problem.
this statement of yours, “can’t quite run it in the Phoenix summers” provides absolutely zero information on what you perceive to be a cooling problem.
WHAT is your cooling problem?
High temperature gauge reading at highway speeds, high temperature gauge reading idling in the driveway, bubbling and gurgling indicative of boiling water in the heads of the engine and if so when and after what driving experience, discharge of boiling steaming water from the overflow tank when the car is parked and turned off??
do you have cooling problems in the summer without the air conditioning being on?
have you used an infrared heat gun to check temperatures of your cooling system components?
Is the 428 a new motor that has always had cooling problems since it was installed?
how old is your thermostat? are you using a thermostat?
are you sure you have removed all of the air from the cooling system?
please share with us the symptoms that cause you not to drive your car in the Phoenix summer.
please share with us what troubleshooting steps you have already taken and what those steps revealed.
Larry