Not so hard to imagine: Say you transitioned from steel line to AN using a tube adapter just after the elbow at the trans. That steel line is now supporting the AN line, unless you tie it up somewhere. Then beat on it around a track for 24 hours, hopping curbs, etc. It may not fail at all, but why take the chance? If you're gonna go AN, do it all the way.
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1988 CVLX Formal Roof (Love Boat) POTM July 2019 & Aug 2020, 56K
E-code lights w/ relays, LEDs w/ switchbacks, 15x8 Ansen slots w/ 255/60 BFG's, Kenwood stereo, various hacks.