Contract with a steel tank erector with experience working on steel tanks. An experienced erector should be able to present his qualifications to do the work. This would include a list of clients, welder certifications, safety plans, etc.
Your choices revolve around doing it in service or out of service. If in service, do it per API 653 section 9.14 including all limitations. If doing it out of service you have virtually no limitations as long as you do it per API 653 and API 650. And of course what bimr said.