I don't have one in a Jeep, but I've had 351's bolted to a wide variety of drivelines and the mileage has always been in the 15-18 range. With a lighter vehicle you might get more, but I wouldn't count on it.
If it's worth doing, then it's worth [I]over[/I]doing.
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The build, the gear, and the mileage: [URL="http://www.jeepforum.com/forum/f22/wasteland-survival-guide-engineering-greta-1344782/"][B]The Wasteland Survival Guide[/B][/URL]