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Technically your not allowed to replace the stock cats with other ones if they're still working properly and the truck has under 80k miles on it. Just pull off the secondary ones and flange the pipe on the one longer side that doesn't have a flange yet. That way you can put the cats back on if you're unhappy with the results. I wouldn't waste my time with high flow cats. Unless you're able to tune the truck for them, you won't notice a difference. Getting rid of the small restrictive pipe outlets on the secondary cats will make a far bigger difference. When I get my longtubes put together, I will be getting rid of the secondary cats to make room for larger primary cats.