That particular company, Special Metals, has a technical staff that can provide you with the information you need. They specialize in stainless steels and high nickel alloys. They are the experts.
Another source for high nickel alloys is Carpenter Technologies, located in Reading, PA. Likewise, they have a technical department that can provide information on the best methods of fabricating, welding, stress relieving, etc.
All it takes is a phone call or an e-mail to speak with the people with the most technical knowhow in the industry.
ASME B31.3 is for piping, ASME Section VIII is for unfired pressure vessels. The Owner is responsible for determining which ASME code is appropriate and applicable. Generally, you don't use the requirements for unfired pressure vessels for piping.
Best regards - Al