IMHO, you're better off saving your money and getting some gears, either 3.55s or 3.73s. 1.7 rockers will increase your lift on a stock-cammed 5.0L by 0.030", but will not create the lopey idle you're looking for.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch 