Dom,
The information you need about the contour of the melt thru on your GTAW welds must be aquired from General Electric I think. There may be several avenues you can persue.
1. The actual class one doccument, which is a specific manufactures specification that will include dimensions and tollerences. This doccument will be specific to a part or family of parts and will be associated (linked in writing) with the specific part number of the componant your fabricating.
2. Your Class one doccument may refer to General Electric's Standard Practices Manual for Aerospace Fusion Welding for melt thru limitations on tube or componants of aerodynamic significance.
3. Both the class one doccument or the Standard Practice manual may reference AWS D17.1 Specification for fusion welding of Aerospace Applications.
I believe these avenues are the only way you can get a real authoritative answer to your question.
Aerospace componant dimensions are often part specific, and manufacturing design engineers may specify tollerances greater or less than "Standard" demensions due to the service conditions of the part.
Once you have looked at these avenues and have some specific data from a reliable source, we may be able to talk a bit more about how to best address your process control issues.