5.8 is a good motor, so is the 5.0. The question is what do you wan to do with it, I kinda sum up my decisions by use. The 5.0 high end power, the 5.8 more of a torquer and as capable as the 5.0. Has the 5.0 been rebuilt because if not you may need to rebuild or place before the 5.8 then the cost of the 5.8 will be offset.
Just my opinion though.
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1984 Mustang GT350 20th Anniversary Cobra
1991 Ford Explorer
1993 Ford F-150 300 I6 5 Speed
1999 Ford Taurus Wagon
"Don't Crush Them ... RESTORE THEM"
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