What is the Temperature of the pavement you are driving on? Are you in the shade? Are you on gravel or dirt? Is the temperature of the engine normal or cold?
All of these things can effect the temperature of the brake drums.
Here in the South temperatures can exceed 190 degrees on the pavement during a summer day. So hot that you can cook food on the pavement.
I do not think it fair to spot the temperature on a brake drum. To determine the effectiveness of brakes. As we all know "hot" brakes do not stop.
The proper adjustment on a drum per my Grandfather and Dad was to jack the wheel up and tighten the adjustment by spinning the tire until it stopped. Then back off until it free wheeled without any resistance. (You can hear the brake dragging slightly in the drum.) Be sure to tighten the check nuts. Each wheel had to be adjusted individually. Take the car for a "test run" at very low speed. Apply the brake to a normal "full stop". If it works properly try at a slightly faster speed. If that works, then at 30 M.P.H. If it needs adjustment it will become apparent during the first or second practice stop.
At 3 M.P.H. you should stop in a matter of less than 6 feet. Distance varies due to road conditions, heat, etc. Having enough room to stop is a judgement call and depends on how much pressure your leg can generate.
You should be able to stop within a reasonable distance.
At speeds in excess of 35 M.P.H. breaking requires slowing down by using gears and breaks.
You said at 40 M.P.H. 15 to 20 feet. I think you have excellent brakes. Is your peddle at the top? With one inch play? I bet it is. Remember that old cars take longer to stop than present day cars, especially at higher speeds.
