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i think it would be a pretty direct swap, 4runner and ext cab are the same frame.
Remember in Japanese Toyote means interchangeable parts.
I haven't gone out and measured one, but the specs I can find show the 84-89 4Runner to be 103" wheel base which is the same as a regular cab short bed. See, for example, http://www.top4runners.com/cgi-bin/i.../1987specs.jpg Xtra cab wheelbase is considerably more.