Fluid Dampener How To.........???

If it is a 70 340 crank it is forged and requires no weights on the converter. Check that out first. The reason your rear seal is leaking is simple. The imbalance has trashed it. Hopefully it hasn't ruined your bearings too. If you have ran it much it probably has ate them up sorry to say. To answer your question as to why a harmonic dampner won't fix an imbalance problem the best thing I can think of is to compare it to a major cut on your head. Bandaids won't keep it closed. Bandaids take care of the little cuts but if you have a gash 2 inches long you need stitched up. Moper said it best with his clarification of us using the terms "balancer" vs Harmonic dampner". It really is a Harmonic dampner, not a balancer. No harmonic dampner will fix it unless you have the wrong dampner on it now.