if your radiator is the 1 you posted the pic of in photos that is NOT your problem its bloody HUGE
i have used smaller radiators on big block chevs
it could be the pic but it looks pretty big to me

check the water flow
check direction of air out of the thermos
check whole cooling system Pump thermostat etc
i must say them thermos are rubish and i find stock fans out of modern cars are alot better that run in a surrounding then them fans im guessing there craig davis fans

i have found on the big v8s the late model commo radiators with stock fans run the best and better then any other radiattor i have ever used

im running a small radiator HONDA CIVIC but double the thickness and an aluminium tyype but if its over heats ill try fit a commo 1 if it fits between cooler pipes