For the record, the new pipe bolted up in exact-fit to where the ancient old midpipe that I bought used from Paul Aber four years ago used to be.
If it is made of stainless steel, it is made of .125" thick stainless steel. Definitely not 16 gauge like I was fearing.
And I welded the O2 bung in at work, and I bolted it on the car, and installed a NEW wideband sensor (the old one was permanently siezed in the midpipe, and also dead), turned the idle screw up a half turn, and fire the car up for the first time in... well, two months. We had a warm winter.
No more exhaust leaks forward of the axle! The exhaust still sounds "sharp" and there's a hole that something poked in the bottom of the muffler shell that I need to address (road debris? who knows) but plugging that with my thumb doesn't change the noise level much.
And as I was doing this the rain turned into snow
I still have to do an oil change and some other misc. maintenance stuff before I can drive it, anyway, so I left it there for the day. It's supposed to be in the 30s tomorrow, warmer on Saturday and in the 70s on Sunday, I'll muck with it on the weekend.