If you convert to steel springs you will need to watch out for bumps in the road because the nose will be sitting on the stops..
Parts are easily available for any 6.9 as Hanno says just ask Neil Dubey .He does rebuilt units for them .
The spring towers in a regular W116 don't exist in a 6.9 as the person who tried to convert Al's car found out . There is nowhere for the top of the spring to sit unless you weld in cups for them . then there is shock absorbers...and it will go on until you run out of interest or money.
Sell the car if you cannot run it,after all these were not only the best cars in the World,they were also the second most expensive car in the world after the 600.... And nothing has changed.