Long tube to exhaust

So I'm looking into a set of Doug Thorley Long Tube headers and I've been hesitant to pull the trigger because I have no idea how it will mount up?  I have a custom exhaust on my truck from the cat back and the headers come with a connector pipe/Y pipe assembly. But my questions are;

Will the L/T's mount up to my current exhaust?

Will I need to replace my cats?

please any help would be awesome! Thank you