What about getting a high flow one for a Mustang or something? It might not be the right size, or you might have to scab it in as you say. But I would think that it would work better than just a replacement one. At least I would consider it, or something like it, an upgrade....
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'99 XJ, 5.5" lift, 33" MT's '11 Dodge Charger
I miss the days that they made toys that could kill a kid.
Now that the price of scrap has dropped, you should be able to get a used unit pretty reasonable. Of course mine has the O2 sensor in it. The rest of piping is in great shape, I hate to not put a factory unit back in it. Some one has one lying around from a part out or HP swap they will sell . A buddy of mine has a complete Flowmaster system with a high flow cat but it's from a 92. I wasn't sure it would be a direct swap into my 96. I need to get over and inspect it.