Overheating Issue in 350z, master tech at nissan want to throw another motor in.

2003 350z, took it drifting at the track few months ago, car started overheating at it was 95 out, bottom rad hose felt cold cut out thermostat got back on track car never over heated again, drove around for 2 weeks with no thermostat no overheating. threw autozone thermostat in car overheated after very thorough bleed. fighting the car for weeks on end ended up throwing parts at it out of frustration oem thermostat oem water pump mishimoto radiator to no avail. countless bleeds seems like air is still getting into system. (yes i pinky promise it was very very thoroughly bleed). did combustion chamber leak down test coolant pressure test and the colored fluid tester 0 signs of blown head gasket. no leaks. took car to stealership, master tech worked on it all this week and couldn’t find the issue tested for HG suggested perhaps water pump was installed incorrectly and wants to open up timing cover,(used all data diy for install and am confident in my work, but this issue still occured before new water pump) he suggested to put a used motor in with a warranty. I am at a complete and utter loss as to what the issue could be I’ve read through all the forums and the nissan master tech seemed to fail in a proper diagnosis even with the $350 diag fee.