Why does my swapped 5.3 run fine idling and crawling but overheat and over-pressurize only under load at speed?

Fine at idle and crawling means airflow from the fan and shroud is OK. Overheating under load at speed points elsewhere:

• The tune: a lean air/fuel mixture makes cylinders run hotter, and ignition timing that's too far advanced or too retarded also adds heat. Speedway notes improper timing or air/fuel can make an engine run hot even with a properly sized cooling system. Load is where a lean mixture or bad timing shows up most.
• Water pump flow and condition.
• Air in the system: make sure the steam line is plumbed so trapped air purges.
• Radiator capacity for the power you're making.

We don't have a source that diagnoses the over-pressurizing specifically, so have the cooling system pressure-tested if the rest checks out.