As long as he's a good and professional welder, he hopefully will know what to do and will do a great job.
What kind of shackle mount did you buy? I always get ones that are a few inches long so I can cut a hole in the bumper (such as in the above post) and weld the back side of them in the bumper as well as to another surface. Definately do not want one of these things breaking off and they can see a lot of shock load/stress.
The ones I get are about an inch thick and I pre-heat them with a torch to help make the welding easier to a 3/16" or 1/4" bumper. That thick metal can really absorb some heat otherwise, and posiibly not get as much weld penetration as it should get (while the thinner bumper gets really hot). I am not a professional welder tho, this pre-heat is mainly to make it easier for me to weld and some more piece of mind.