Skaggy dog
I have fabricated some spool pieces like this before. The most time consuming aspect of the project was positioning for tack welding, and tack welding itself. Holding the components square, rigid and true while assembling, tack welding and cooling was quite time consuming and challenging. Cleaning of the tack welds was also time consuming.
For the actual welding of three thousand pipe spools, and the joint indicated, you should consider FCAW. In addition, I recommend that you RENT a small weld positioner with a variable speed motor. After you make a template to center the spool piece on the face plate, vice grips can cut up and modified / welded into a configuration suitable to hold the parts in tight contact for welding. Most positioners have a work lead lug to attach to the earth grounding and the welding machine, so that you get good electrical contact through the rotator face plate.
You have not described what testing these spool pieces have to pass. Short Circuiting GMAW will have more start/ stop Lack Of Fusion (LOF) defects than FCAW, and if any kind of leak testing / hydro-testing is required, the time spent in repair and re-testing, will eat up any savings garnered by short circuiting GMAW.