Jooseppi, glad to see you are getting a lot done!
Interesting way to make the W123 transmission work in a W126! Too bad the tail cone swap doesn't work.
As for trimming, I would tape a piece of medium grit sand paper onto the work table and rub the edge down until a perfect fit.
I suggest you put the Dremel 3000 on your wish list!
It is the best power tool investment I have made! Sliced off the rusty stud so I could remove the battery tray and removed loose rust scale. Hope soon to buy the attachment kit, get some polishing compound, and correct the scratches the body shop put on the mirror glass. What I love most is that variable speed so doesn't scare me and go to fast to where I do damage.