My newly rebuilt engine is humming like the day it left the factory but of course it is only 99% correct and I've read enough threads here to know that 1% is most bothersome to my brethren here .
It is fairly common to have the water pump boss numbers planned off when the block is machined. A friend let me borrow a character/number stamping kit so that I can imprint the serial number back on to the block with the warning not to hit it too hard and chip the block. I am having a hard time with that first hammer blow. Has anyone done this before? Advice?

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.

Your not going to damage or hurt the block casting by hitting the metal stamp hard or double hitting it. What I do is use at least 1 lb hammer and hit the stamp hard. If it's not imprinted deep enough I carefully put the letter or number stamp in the same imprint and hit it again. It's important to get it exactly in the same imprint before hitting it again.

At least you know what your numbers where, my block was decked by a previous owner