I'll share a little bit about the setup I have in my Cutlass.
I used the jegs long tube headers. They fit pretty good but be forewarned they are cheap headers. The coating will burn off after the first run, the welds aren't the best, but they don't leak. I'm not a fan of the 3 bolt collector.
https://www.jegs.com/i/JEGS-Performance-Products/555/30060/10002/-1
The headers are mated up to the Jegs 2.5" exhaust kit.
https://www.jegs.com/i/JEGS-Performance-Products/555/30556/10002/-1
Fitment is pretty good, pipe quality is also good. It is all mandrel bends. Keep in mind this system does not come with any crossover pipe (x or h). I added an H pipe to mine. The hangers that come with the kit aren't that great, I've broken a few and replaced with something better. Mufflers are easy to swap out with this kit, I have changed them a few times. Another thing to consider with this kit is exit location.
Here is how it comes out of the box:
View attachment 87976
Or you can cut the last bend off and end up with something like this:
View attachment 87977
I should also mention I am using the jegs cross member. I have no exhaust clearance issues.
https://www.jegs.com/i/JEGS-Performance-Products/555/640601/10002/-1
This Pypes exhaust kit sounds like it may check all the boxes for you.
https://www.summitracing.com/parts/pye-sgg10s/overview/year/1985/make/buick/model/regal