what year 5.0L or 5.8L

Unit is correct, Those are SAE gross horsepower numbers so they come out to be approxamately 15-20% less depending on trans/accesories, etc. to be close to SAE net., but yeah, those earlier blocks are stronger and would produce quite a bit of power.
I read this site where this guy with a 351 and mild cam/intake/carb and stock exhaust manifolds -from the 302- with dual exhaust ran low 14's with his otherwise stock early fox mustang (think it was an '81 or so). But it's a site on Geocities.
I personally want to build up a stroked 351 because i don't like the idea of revving really high or using nitrous/forced induction (at least on a carbureted car) to get the power of a big cubic inch engine, which would probably be the most reliable route to go. Just my opinion, good luck loomis!

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'84 Mustang 5.0 LX
My car