Go get a book "Process Equipment Design" by Brownell and Young. It has the very old code included and allowable stresses of those old firebox materails are there. so no guessing.
Carefully google or using Amazon.com or ebay you can see the entire content of the book, but not allowed to print. However, you will find the allowable stress in the middle of the book. If lucky, you can still buy one for less than $20.