So Mr Toops, the harmonic damper is one of the most important yet ubiquitous parts of a straight 6 - often mistaken as just a drive pulley. The crank experiences large forces at either end and being long goes through torsional distortions. It also has a natural frequency that can resonate at certain RPMs.
If you consider the firing sequence of 153624- cylinder 1 is pushing down hard while cyl 5 at the other end of the crank is compressing.
Now factor in lots of revs and the "harmonics" that build up as a result of the other cyls firing and it becomes apparent what an important job the damper performs. A couple of great articles below:
http://www.atiracing.com/products/dampers/101/index.htm
http://www.atiracing.com/products/dampers/damper_dinan.htm
Of course this becomes all the more significant as you raise compression / torque and revs, but harmonics are always there.
I would always advise loosening the fan belt and checking the damper bolt torque as a first step. Mine appeared tight but was totally loose (1cm!!!) on the old smokie. Then if able, check the general condition. On tinter, I've not come across stock dampers that have failed on stock engines but maybe I haven't looked hard enough. At the end of the day it's an insurance policy on your engine and who knows how well 40 year old rubber is performing after all these years, miles, heat cycles etc.
The stock dampers are NLA. But MSA do some interesting alternatives. The cheaper ones have been known / reported to have come apart and definitely avoid the Chinese knock offs of those on eBay! The consequences of a poorly performing or broken damper can be expensive.
https://www.thezstore.com/page/TZS/CTGY/classic12d02c
BHJ do a nice looking bit of kit but as one of the US guys recently posted they rust faster than a 70's Datsun on salty roads.
This photo I believe was less than 1 year of use ...
Kameari and ATI do great dampers (the latter only comes with a racing sized pulley). But both come in at around £400-500 delivered.
They are both around 13kgs in weight.
If you do go this route go Kameari street fluid damper. Zero maintenance and street sized pulley.
I will let others more experienced than me chime in with their recommendations.