12 gtx 215 overheating

[Hilde33]

Just had engine rebuilt about 3hrs on it since only 35 on unit total timing can broke. Since rebuild the unit is overheating but only does so if you ride it near or at WOT it's pumping out of the over flow hose from the tank today we put in bit left on trailer and ran unit for near 45 minutes with zero issues also holding rpms near 4500 then took it off the trailer ran unit at WOT foraybe one minute lifted set and coolant is coming out of overflow also have pressure test the coolant system at 13 psi for about a hour with no sign of any leaking or pressure drop. What am I missing