Pretty sure it's around 600hp you need to go to a 3.5" aluminum. That's what I have, and got it through classic chevy and inland. They recommended it for my application. My exhaust is tucked way up and have no issues with exhaust contacting anything. They did modify it some. All you will need is some stainless 3" pipe and a good welder. A call to Jim at Classic Chevy is a good idea. He is extremely sharp. I like to be well within the rating of all my parts...especially a drive shaft. Reliability is important.
PST and Mark Williams both said I am probably okay with the 3" mild steel that comes with the TKO trans, as long as I have the stronger 1350 yokes/u-joint stuff.
If I don't, PST recommended mild steel (up to 1000hp) and MW recommended I might as well go with 4130 chromoly (what JP has I assume).
Now I just need to figure out what I have on my 9" and go from there.
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2004 NASA AIX Mustang LS2 #14
1964 Lincoln Continental
2014 4 tap Keezer