Maybe you forgot one of the clutch cover bolts? Other than that, I think only a bent crank would cause THAT bad of a vibration! Eh, mebbe a harmonic balancer, but I doubt it. How's the pilot bearing? Check firing order as well. Good luck.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch 