I'll be honest, I am disappointed by this development. In the end, the only real reason I abandoned my turbo project was the half-assed downpipe. I sold this kit to the wife as "everything will bolt together, I will not need to weld a single thing." I was honestly expecting something that bolts to the second cat. And when I heard this thing was going the way of CARB, I kind of assumed the bolt-on downpipe would have a new cat in it, or there would be some sort of exemption to run with only one cat. In my opinion, essential to the "anybody can bolt it on at home in a weekend" theme of this kit is the idea that 1) you do not need to permanently destroy any parts of the truck (stock y-pipe), and 2) you do not need to weld anything. You're also trusting the end-user to cut their y-pipe perfectly aligned with the flange of your DP. I am a handy person with a lot of wrenching experience and just a little fab experience, but I have already proven that task is fairly easy to botch.
I have bought 3 y pipes in the last 2-3 years, one, which I chopped up to make a downpipe much like you are having us do here, one which was needed to replace the chopped one when the turbo came off and was subsequently smashed off road, and a replacement for that one. Now with the real Magnaflow pretty much NLA, I am not going to chop this one up. Which means I'll need to source yet another y pipe or just fabricate my own lower section/cat. Which ultimately leaves me with the exact same weakness of my original turbo kit, and the only reason I dumped it in the first place.
At this point, I am invested in the progress of this kit, but this is literally the ONE THING I was trying to avoid by getting the kit. I'd pay more for a complete downpipe that bolts to the second cat.
Sorta just venting here, but I also do hope you're not too far along to take this point of view into consideration before the final product.