The stamping process is pretty simple. But if you are worried about how much force to use, find an old piece of cast iron to try it on first. If you don't have any cast iron to try it on, try some old pipe fittings or a pan or..... With an appropriately sized hammer (16-20 oz) it won't take much of a hit to make nice deep numbers. Don't forget the CJ or J3 and the dash as well if it had one originally.
One hint....try to rig up some sort of alignment jig so your numbers aren't really out of shape looking. If you can make some sort of rectangular opening in a piece of wood or metal to keep them aligned in a row , practice with it to get the spacing right, or make something to set the spacing as well. Hand stamped numbers really look bad when done carelessly. Also try to match at least the size of the originals. Getting the right font will be tough. I keep trying to find old stamp sets that are right but so far the search is futile.
You might be able to just put a piece of wood like a paint stirrer against the head to hold the stamps a uniform distance away from it.