[Bluesman]

I too had overheating gremlins. It started when I moved
from the cool coastal region to the central valley where temps
get well over 100 degrees.
The radiator that comes with the Butler kit (at least mine anyway)
is a 6 core and thick as a brick. It works fine at 70 miles per hour plus. You then, with the help of a shrouded electric fan, could actually get air all the way through it.
I installed a simple (thinner) Griffin aluminum 2 core. I think a 22 X 19" is the correct size. 160 degree stat. If the water pump transfers the coolant to fast through the system, this also will not allow the coolant enough time to cool through the radiator.