Similar prob with 440 in a 69 Coronet. Moved the trans cooler closer to the grill. Stacked like you have them added to high heat area with too much flow restriction. I had the headers thermal coated inside and out to kill a bunch of under hood temps. Steel hood had a pair of Superbee scoops mounted when we got the car so I used a hole saw to let them move a little cool air thru the engine compartment. I believe these cars have such under hood high temps that it becomes too difficult to dump temperature from the radiator into a compartment that has a higher temp. Same radiator as before but much better manners.