I found out why the welder was giving me fits. It had a spool of .035 wire instead of .030 wire in it. I pretty much never use .035, but got some for when I was working on the crossmember. No wonder it was so hard to control when welding thin exhaust pipe. Switched up to .025 today, and it made my life a lot easier.
Spent a lot of time cutting, grinding, test fitting, cleaning, and welding the passenger side midpipe. Needed adapters to get the diameter large enough for the Sierra's cats, so took a lot more pieces than I would have liked. Note to my future self - next project should be something that didn't come with catalytic converters from the factory, so I won't feel guilty if I don't add them. This would have gone a lot faster if I didn't use the cats, or weld in O2 sensor bungs for the downstream sensors. It is all tacked together now:
I have it under the car, propped up on jackstands, so it'll be easier to test fit the driver's pipe, as space is tight enough the cats have to be staggered.
Just as I was beginning the driver side pipe, my abrasive chop saw decided it was time for the blade to wear down enough that it couldn't cut all the way through 2.5" piping, then my grinder decided it did not need the ground connection on the plug. A trip to the hardware store for blades and a plug turned into picking up an early dinner, and then losing access to my work clothes for a bit, as eastsideWife took over the bathroom they were in for a while. Since I lost a lot of daylight, I tried to do this pipe a bit more efficiently, and it may have bit me. Like the passenger side, it needed a bunch of adapters, but this time, I cut, ground, cleaned, drilled, and welded everything in one big batch, rather than a little at a time.
I still need to add one more 90 degree bend at the front, but I need to do some test fitting tomorrow. I meant to angle the O2 sensor bung slightly towards the passenger side. At the angle it is now, a sensor or its wiring may have interference issues with the driveshaft. If so, I'll grind the welds on the 90 elbow, reclock it, and reweld it. Really hoping it works out, though, as weather is looking bad tomorrow, at least early in the day.
At the very least, both pieces can be test fit, and I can see about installing exhaust hangers tomorrow. May need to pull the passenger seat and drill another hole or two in the cab floor.