Made some good progress on this today.
Not thrilled that I have to neck this down from 2.75" to 2.5", but there's only so many sizes this stuff comes in. I doubt it'll make any real difference in how it performs.

It's more about the annoyance about having to cut down the various adapters. The muffler is longer than I thought it'd be, so I have less room to work with than I was expecting.
Measure, measure, measure, cut, measure, weld. It's not going to win any awards, but I'm more or less happy with it. Especially since I free-handed the cuts with my angle grinder.
Tomorrow I'll measure up the back segment, cut out the old silencer, and weld the muffler to the back section. Then I'll figure out how I'm going to route the exit under the rear bumper.
I mocked up the y-pipe and muffler and started it up tonight. It sounded pretty good. I think I made the right choice with the muffler.