Erich,
It's a good question you raise. But what about the famous '64&1/2 Mustangs,
the ones sold between April 17 and mid-August '64 which was available with
the 260, the 4-barrel 289 V8, or the 289 HiPo? Obviously, these all had
5-bolt bell housing patterns. I have even seen it stated that the early '66
Mustangs had 5-bolt 289's, which makes sense if they continued production of
this version until August '65. The change from a 5-bolt to 6-bolt is easy in
most cars like a Mustang since there is a much greater availability of
6-bolt bell housings than 5-bolt. Maybe you should post this question to the
Mustang Usenet group and see what they say. For example: "When did the
6-bolt 289 first appear?"
TTFN,
Bob
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