I tried a 3" X pipe on my Barracuda when it still had 340 manifolds on it. I do not have back to back times on it, but my conclusion was that it was contributor to the lack of torque and difficulty with getting a decent idle. Now that car has headers based in large part on Calvin Elston's recommendations.
Calvin Elston calls the gas flow "king" and explains it here:
"What is [exhaust] Blowdown Length?" p.2
He's been pretty generous in sharing his experiences. This is one about merging to a single tailpipe.
Flowmaster Outlaw muffler - anti-reversion? - Page 3 - Speed Talk
He done some slant sixes. He took down his website, but maybe I can find some pics because a few years ago I was emailing with Lou about them