The only restriction on bead width that I know of in D1.5 is found in 4.4.7 and Figure 4.1. Unlike ASME or other codes, I have not see any other restrictions.
The acceptance criteria for root and intermediate passes is not spelled out specifically but is implied. Naturally you wouldn't want to weld over porosity, overlap, slag inclusions, and so forth. But concavity and undercut that would be unacceptable on the cover passes could be welded over as long as the profile is not likely to cause problems. Just remenber that if you have excessive concavity, you could encounter some cracking. It's better to have a bit more throat to prevent that but keep 4.7.7 in mind also.
Every process in D1.5 except low-hydrogen SMAW requires qualification prior to use so I assume you have done an unlimited thickness 3G PQR within the last 5 years for the GMAW? Your production welding technique would be the same except that you likely would not have welded the root with 7018 (that's not a problem, you can mix approved processes).
Chet Guilford