I've recently replaced the lever for a slightly longer one. Can't tell for sure if this had any effect, I tend to think it did but that may be wishful thinking.
I do have found another even longer lever, but the indexing flat makes it angled way different then the other 2. I probably have to alter the KD-linkage for this lever to work.

The current KD-linkage is stock '73 as far as I can tell.
The car had a 904 when I got it, which I replaced with a OD-only A518 later on, and now recently I installed a 518 with OD & LU.
The first A518 worked fine with the current KD-setup, but didn't have a TransGo-kit installed or anything.

The 'drum-seal'-issue mentioned above has me thinking. Anyway to test or rule out this issue?
When putting the tranny in Reverse the engagement feels very soft by the way, if that proves anything.