Hi Shebg,
I think your C263 (Also known as Nimonic 263) would most closely fit into group IIIB Precipitation Hardening Nickel and Nickel Based Alloys. **My Opinion**
It has similarities to both Rene 41 and Waspalloy in that regard.
Understanding the pre-weld annealing and post weld solution annealing prior to aging is a far more important factor in your work than placing it in the D17 grouping in the WPS.
The weldability is quite good, so long as the rigorous process engineering controls are researched and implemented.