I'm not gonna be much help but here is my thread with pics
This is the part i would pay attention to in 96black post
(especially the OTA which a real PITA with any 2 1/2".
This is where i had the most pain.
Mine rubs on the sway bar when you hit a pothole.
It's hard to give advice, but get car on lift and look for the hard spots and easy spots to make alterations to the system and go from there.
I don't know exactly what Dan does, but he claims on the website that he intentionally makes his OTA pipes differently than either the stock or the other aftermarket stuff out there so that the 2½" will fit easier. He can also supposedly do 3", but I don't foresee ever being at a point where I'd need that much exhaust flow. And I think with the 3" his site does say that clearance can be an issue, though I'd have to go back and read the listing on his site again to be sure of that.
The sway bar is interesting, don't think I've heard anyone mention hitting that with their exhaust. I did look at your thread and watch your video, and if you have a true Impala I'm thinking a part of your issue there may be the lower factory suspension (assuming it hasn't been lowered further with aftermarket parts). I suspect that would leave less room between the rear axle and the underside of the car.
If your car is a Caprice made to look like an SS and still has a stock height suspension, then I'd be a little more concerned since I think the stock Caprice and Roadmaster suspension has nearly the same ride height as the Fleetwood. And at this point, I really have no desire to lower my car, if anything I'd like to raise the rear up a tad, give it a little bit of a raked look.
I'll just have to see what I'm working with once Dan gets everything made and delivered to me. I used to have access to a car lift whenever I wanted, down at my dad's shop, but I'm not really talking to him anymore. Long story, one of those you can pick your friends but you can't pick your family deals, he's got his demons and until he's ready to work on that, I'm out. So I'm stuck working on everything in my driveway on jack stands.
Maybe while I'm waiting on Dan's stuff I'll get ambitious and finish stripping the junk car in my garage and get rid of it. It's a little cramped in there, especially with such a big car, but at least then I can work inside in the A/C.
Thanks to everyone for their suggestions and input so far. And perhaps we'll get lucky and after guinea pigging this thing with me, Dan will have the specs to make a perfect catback system for the Fleetwoods.