I have done a steering wheel replace on my 1971 350 SL. I put a W 126 (1985)wheel on the car, but as you say, the splines are diffrent. The W107 splines are VERY fine ( 14,5 mm diameter), and the W 126 have different splines and 20 mm or so in diameter.
I buy a Mercedes aluminum boss,(or adapter) from my local supplyer, (sorry my bad englich) Turn it down in a lathe and thread it outside 1" UNF (25,4mm)
After that i put the W 126 wheel in the lathe and thread it inside 1" UNF.
Secure the "new" boss with lock tite and four- 5 mm screw.
The new wheel are great.
From Norway
Arvid
W 107 350 SL 1971
W 210 E 240 1999
http://www.chevroletnorge.com