When I drove my 1969 Swinger 340, 4 speed, with a 3.55 rear end gear, on the expressway at 75 mph (3,500 rpm), it overheated at 220 degrees, in only 2 miles. Driving on back roads at about 45 mph, the temperature gauge always remained at about 180 degrees.
The 340 was rebuilt a couple of years...