If you are working to a code, you would not only need to have the procedures approved by a Level III in those methods, you also need to have your own certification procedures.
If you are performinmg the NDT yourself, you would have to have a procedure within your company either designating a Level III or designating an outside agency to certify you and your procedures. You can use an outside agency, as long as they meet the requirements of SNT-TC-1A or whatever document you are working to.
Taking a course does not certify you as a Level II.
You must first gain documented experience and then be certified to your comapny's procedures.
This is a requirement of SNT-TC-1A.
Of course, that all depends on what your client will accept.