I almost guarantee that it is the harmonic balancer! It will make the car shake like hell. Sometimes it wont visibly shake but could be slightly bent. If it vibrates in neutral and only related to engine rpms, it's the balancer.
Or.....
I once had my plug wires re-arranged by an enthusiastic but unknowledgeable mechanic and he had changed the wires to some old 70's ford firing order. The car actually ran, but it shook and ran like @ss .
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91 GT Carb conversion, holley 600 double pumper, edelbrock performer intake, FMS "C" drop springs, march 1000 underdrive pullies, crane 1.7 roller rockers, GT-40P headers, bassani x pipe, american thunder catback, FMS 4.56's, msd aL6, trunkmount battery, A/C eliminator kit, 3000 stall tci streetfighter, AOD with transgo kit, A+ servo, 300M hardened lockup shaft, kevlar bands and 28,000 gvw trans cooler, 3 core radiator, 300 lbs stripped with a full interior
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