Are you sure you didn't unplug a vacuum line somewhere while trying to put them on. Check your firing order. Even if you were to run it on open headers it still wouldn't shake. I don't think the headers are at fault here. My best bet is what I stated. Sometimes while trying to sneak them in you'll unplug a hose.
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67'fastback- Edelbrock carb, intake, cam, valve springs. Road lifters, Comp Cam rocker arms, windage tray, cerama coated headers, dual flows with H-pipe, modded c-4, tci stall converter, perma cool electric fan, aluminum water pump, griffen aluminum radiator, accel 8.8 cable, wide cap distriburator, magnetic pick up, manley valves, v-matic hurst shifter, edelbrock performer nitrous, carter electric fuel pump, ignitor coil, solid motor mounts, magna racing suspension, poly urathane bushings, und
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