Hi
I did it to mine, but I have a CRD version of the 300C so I have a intercooler (larger then OEM) so I bolted with wide bolt rings directly to the intercooler, I have driven for hours at 120 MPH, no problem. However.....
the first time I had the fascia on the mechanic did not do this, once I reached 100 MPH I heard what sounded like I had run over a box and it was stuck under the car also he did not secure the inner fender lines with push pins (he used the old ones) so the wind pushed my liners against the wheels which eventually rubbed a hole , had to replace the front belly pan and inner liners, but now its secure, I think the instructions say not to put the front belly pan back on?