Brake pedal is really stiff

OK, you clearly did not understand what I'm getting at. You can't just throw your second set of drums on there and really tell anything, unless you get incredibly lucky. If they are worn out some, they might just have the very same problem

Once more, THE ONLY WAY you can see if the drums are oversize or worn badly is to remove the shoe from the car, and place it in the drum where you can SEE it and see how it fits, critically. REALLY take a look at it and see if it fits in there tight, or whether it "rocks."

I realize you probably don't have a lathe, but you'll have to take at least a couple of the drums somewhere that does. Just tell 'em you are having poor braking troubles and you want to know if they are worn oddly or turned out too far. They can "mike" em and check them. You can check for several wear conditions by putting a straightedge across the drum surface. Trying to check for "bellmouth" is a little harder, that's why a lathe works good. If you have a good combination square, you can get a good idea by laying the straightedge across the back of the drum, and let the square shoe down inside against the wear surface.

Here's the thing: Manual drum brakes have been around a VERY long time, and they don't operate by magic. There are strict known principles by which they work, or don't. What you are describing sounds exactly what I've been harping on --poor fit of shoes and drums, or glazed or poor quality lining.