PE:
While the joint is prequalified (B-U4b) using a single bevel and backgouging, the material itself is not in accordance with the D1.1. I have a letter from the former D1.1 chair stating if a material specification is not listed in Table 3.1, it is not a prequalified material, period. (sorry about the redundancy...)
If you are the EOR, you do have the authority to waive testing in accordance with AWS D1.1:2002, 1.4.1. You will also get to answer for any post-welding related problems or issues if any occur.