Miguel- The post you linked to has the finished diffuser mounted on a Peugeot 905B HD body wide pan car.
This is the mod you are talking about but it is on my World GT narrow pan car with Nissan R34 GTR body. I took the rubbery Associated bumper and cut a curve to the front to match the diffuser and give me a good gap all the way accross. It worked very well last test session above. There is a piece of CRC stiff foam shaped and beveled on top of the front of the bumper. The two pics show the before and after. The left picture worked just Ok. The second picture worked well and kept the nose planted on the straight.
Yes this is to prevent flying into the air; it prevents blowovers.