Paul, The DOT (US Department of Transportation) process varied greatly, depending on what state you live in, even what county, at least here in California
One of our locals who owns a Euro M635 registers it in a county that still doesn't require smog checks, if you can believe that
But he's on borrowed time and he knows it
The process did require reinforcements to be welded behind the bumpers, in the doors, additional side marker lighting, US headlamps, MPH speedo, even the word "Light" had to be changed in the dash where the headlight switch is
We see all sorts of conversions still to this day here, some better than others

Bobbo
1980 633 CSi Cypress Green/Pearl Beige
2017 X5 M Sport Xdrive 35i Carbon Black/Ivory White
2005 330 Ci ZHP Cabrio Imola Red/Bone/Black
