I can't imagine that the only 3 speed automatic available was a TH400 (Option M40). There's no way a 350/255hp needed a 400 behind it, and I can't imagine that all 3 speed automatic 69 Acadians were TH400's. . I wonder if that was written up before M38 TH350's came along?
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1966 Strato Chief 2 door, 427 4 speed, 44,000 original miles "FAKE_66"
Carl, the M40 RPO was a generic option code that at that early time just was referring to the 3-speed automatic. They were all TH350s in Acadians as there were no factory Big Block versions requiring the TH400 as there were on Novas.
It is a similar situation to M20 representing 3 different transmission in a '67 Beaumont; 2 different ratio Saginaws (283 & 327 versions), or the Muncie behind the 396s.