[watahyahknow]

its a bit hard with the 400 crank it cant take the 350 stuff because you cant realy balance it internally and cant take the 400 stuff cause its too mush imbalance

they used a rattler or fluiddampr for these aplications but i heared some bad stories bout the fluiddampr one later on .

if the machinist balanced the total rotating assembly (damper crank and flexplate combined) it might be it trown a welded on weight of the flexplate .

the weird stuff the tacho does at 3000 rpm might allso point to a cause maybe the weights in the distributor are loose or theres a vacuumleak or just a crack in the distributor cap
since its happening while not in gear it shuldnt be too hard to hook the engine up to a oldfasion testcase and see wat the ignition does it isnt too invasive on the engine and should rule out the ignition if it isnt the culprit