Are you 100% sure it's extending all the way? It should be plenty long enough for you but who knows what's been done to it in the past. I would go to a salvage yard and find another one (or find someone parting out an SD), should be ok from any W126 300SD.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k