[watahyahknow]

hmmmm interesting , i had seen a few youtube clips with windowed L6 blocks that had spit out a rod , turns out it makes it verry hard to get the block out withouth the trans and the mecanic opted to lift the whole body off of the chassis

didnt known it was because of the harmonics , my guess is they downshifted manually downhill and got intoo that 6500 range hitting and passing the limiter

how bout one of those rattler ballancers or a fluiddampr these should work verry whell for that sortah stuff
not sure if the taper and general size of the snout would be the same as an internally balanced small or bigblock , that might make the choice of balancers a bit better

as for the long chain , it depends how whell the tensioner and sliders are at both ends of the chain it prolly will slap around a bit at that rpm
think the best solution would be to replace it with a beltdrive
might be that the chain is actually causing the harmonics in the first place