Are there 6 cylinder large tachs? I have never seen one. The tach needle is freely adjustable on its shaft, unlike a speedo needle say. Set it to bottom at zero when off, or in your hand. With a little trial and error you can match it to a test tach reading or with a believable idle of 850 rpms or so. You can't read it that accurately (+/- 50 rpms) anyway. The flutter may be another problem but the hookup should use the red/white in the dash of any 84.