Replaced my 5 cylinder Volvo V70 2.5l turbo engine with a 6 cylinder S80 2.9l twin turbo engine and I'm having trouble with the exhaust

Replaced my 5 cylinder Volvo V70 2.5l turbo engine with a 6 cylinder S80 2.9l twin turbo engine and I’m having trouble with what to do about the exhaust.

The original 5 cylinder had only 1 turbo, 1 pipe, leading to one big catalytic converter. It had the one O2 sensor before cat and one directly into top of cat.

The 6 cylinder has twin turbo therefore has 2 pipes, each with its own cat followed by the pipes (1+1=2) conjoining just before I’d say a medium cat. Now, I know both pipes have an O2 sensor before its respective cat.I want to say that the other two sit just after the med. cat.

Other than the original large cat, the other pipes I purchased at the junkyard. I actually found an S80 with the two cats I’m missing and they took them from me at checkout as it’s illegal for them to sell them to me. You couldn’t imagine how infuriated I was and look what I’m having to do now!

So to my question, should I use the single large cat only and weld in an extra O2 housing on top of the cat so there’s two before and two in cat?

should i say to hell with the cat, straight pipe it with the 2 into 1 exhaust pipe i have acquired from a completely different S80 non-turbo? I can use a small piece of the 2 into 1 two conjoin the two exhaust pipes connected to the engine with the original catalytic converter pipe and then worry about O2 sensors in the cat, welding a second 02 sensor housing into it.

Now that sounds more reasonable than simply straight piping it and having to worry about check engine light. Or since my other option is altered (two missing small cats, and would make a second O2 sensor hole in cat) will I have to worry about a check engine light anyways?

I think that about sums it up. Thanks for any advice in advance, community!