[vector1]

imo

usually 40mph+ should be driveable without fan running.

your cooling fins are not plugged, you can see clearly through the cooling tubes?

using a shroud on your cooling fan?

radiator ducted to front opening?

air as much as possible removed from cooling system?

bypass hose used or holes drilled in thermostat?

air moving through the radiator must have someplace to escape to, preferably lower pressure area, such as wheel wells. is your engine bay sealed off from the wheel wells keeping the air from moving freely through the radiator? especially if you have a hood scoop and are pressurizing the engine bay.

overflow hose routed to bottom of overflow tank so when coolant is evacuated it will be recovered?

thermostat in good working order?

just some things to think about. hth