Hey Ian,
Don't know about mounting onto the cross brace assembly in front of the radiator as not sure there would be enough room. I have mounted this type of add on electric fan on a BN6 and on a BN2 and mounted directly to the radiator using the plastic pieces that push through the fins. Pretty easy to mount and only downside I know of is you obviously cut down on the face area of the radiator for air to flow through with the fan mounted. Of course, you then have the electric fan to push through more air so it's a plus overall. Before I added the fan my BN6 would run over 200 degrees very quickly if I got stopped in traffic or at a light, now keeps at 190 which is an okay temp.
Regards,
Mike