You can see how there are different sized rivets on the cab of this Northern Pacific W-1.
With a small hammer I tap an indentation into the top of the brass capped plug with a steel rod that has a point filed in a lathe chuck. This point will be to the head size of your rivet project and when tapped on the brass plug this will be your male and female die. And no, after punching strings of rivets on your tender sides the brass plug indentation doesn't wear that much so that the rivets change size down the row. Note in the pictures of my tender sides that they all look the same. I have even built a tender for a 2-10-4 that is larger than many box cars. It has very large slab sides. When getting towards the end of that much punching, or pressing, take your pick, no size change could be noted.
When you have the rod point to your size, and the indentation hammer tapped into the brass plug, tighten the rod in the drill chuck, get your eye down close to them and carefully align the X-Y table holding the clamped block and plug, with the rod point. While holding the lever down with the rod point in the dimple hand spin the drill chuck to further zero the alignment. Now file a recess the distance from the dimple indentation of what ever the spacing is in your rivet line. (See drawing) This recess is used to space the rivet impression by moving the sheet along the fence and letting the dimple drop over the edge of the filed recess. Set the fence to the correct edge distance and run a scrap piece of sheet brass, or styrene, through to check the size and spacing. Is the indentation too small? Chuck the rod and file another point slightly larger, round it a bit, whack the rod again in the same indentation, align rod point and dimple, and try it again. Once the set-up is done you can press rivets with a surprising amount of speed.
If you have a different spacing for the same rivet head size you can file another recess opposite from the first and run the brass sheet in the opposite direction.