Mark, I have never heard of someone actually welding a steel floor plate into Rem 700 to render it a single shot. I am not sure what the purpose of that would be, considering the damage that could be inflicted on the action.
Now, what is done is to epoxy a aluminum follower into the action. These can be obtained from any of the large supply houses, such as Bruno or Sinclair.
Keep in mind, about all this does is allow you to shoot single shot without too much hassle. Any "sriffness" added to the action would be so neglegable that you would not notice.