Hello Thor,
the M-B wheels did not need any technical modification to be fitted on the Bristol, as the fixing holes and offset were spot on The big job was drilling (obliquely) three holes on each wheel to fit the studs holding the hub caps.
I can add that I wanted to fit steel wheels to try and keep the original look/style of the 409's original rims, but there are also some splendid Mercedes alloys that, in 6Jx16 size and 46mm offset, would fit equally well and are also quite nice to look at as well, as these 9-spokes wheels:
Original Mercedes 16" Alufelgen A- B-Klasse W169 W245 - eBay (item 260788453466 end time Jun-21-11 06:24:06 PDT)
Cheers
Stefano