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catch me if im wrong but all 400s should be externally balanced. If its a 400 block then it should be externally balanced, with a specific harmonic balancer and weighted flywheel. certain 383 strokers require the ext. balanced setup when using a basic 400 crank and not a special stroker crank. i went through all that while doing my 383 build, i had to get an old impala 400 balancer and make a 153 tooth ext. balanced flywheel since i couldnt find one after a while of looking. however, it still could be a 400 block with an interanlly balanced setup(not stock) that would be if a 350 crank was installed to get a 377 destroker motor, known for high reving. These motors arent too common and i doubt thats whats in there, but just thought id mention it.
most likely i'd say the vibration is due to that the weights fell off or were never on the flexplate/flywheel. check that first, should be possible by just removing the dust cover/inspection plate behind the oil pan and looking at the front side of the flywheel/flexplate. crank the engine around with a wrench to check the whole flywheel/flexplate. if they are there, check to make sure it has the correct balancer. If you need to buy one, ask about an older Impala SS with a 400, that harmonic balancer will fit great with the serpintine belt drive on the 88-92 engines(what i used it on) and also the style from 82-87. hopefully this info helped some.
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