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With the right skill, patience, time and money you can do anything. The 65-66 and 67-68 are different widths so it would not be an easy, fast, or cheap mod.
I saw one of those on craigslist a month or 2 ago for around $75, I can't imagine anyone doing that to a Mustang, except maybe Sonny & Cher (old guys might remember theirs).