Make sure he is not a butcher! Cut the floor out yourself and take your time and do it correctly. Most people who are willing to do it for you get in a hurry and don't take the time that is required to do it correctly. But if you have the hard work done and all he has to do is weld you should be ok. Make sure you use weld thru primer in areas that you will not be able to get to later so you don't have rust coming back in a hurry. Remember that when he welds it should be one spot weld at a time, at least an inch apart, and should not be a long weld that will warp the sheetmetal.