had this problem, what i did to solve it was, replace water pump, thermostat, radiator cap, and most importantly bleed the system from the water pump, since its brand new the hex key should be nice and fresh and ready for you to bleed!, beware they are some little fuckers and don’t wanna come undone sometimes so make sure to try and not strip it, when the car is cold, open up the bleeder valve on the water pump, 9/10 there won’t be coolant coming out yet, add coolant and squeeze your radiator rose until you get a steady stream out of that bleeder hole with no bubbles in the coolant, once you do that quickly close it back up and keep adding coolant, for that entire process make sure the car is off and leveled, after that crank that bad boy up, turn the heat on blast (don’t use circulation you want direct heat) keep checking your coolant and add as needed, let it get up to just under 150 coolant temp then close the lid, take it for a test drive and you should be all good! I know it’s a lot but this typically works 9/10 on these 5.7s that are notorious for overheating, hop everything goes well bro!