Requesting cooling/radiator/fan help for a slant six

There were two different width radiators for the 66 "A" bodies. I forget the width off the top of my head. The standard slant six radiator was smaller (it had a large mounting flange on the pass side. The heavy duty radiator was wider. The core was the width of the core support opening, and had a smaller mounting flange. In your case, I think the standard radiator would work OK. The larger HD radiator would give you an added safety factor. Just make sure either radiator is in good condition.

PS: I would fab up a shroud, to increase the effect of the fan. Ever see a clutch fan without a shroud?