Another update:
Today I worked from 9.30 AM till 10.15 PM on the car. And I got to say that it started to look something (the chassis Smile ) but I sadly didn't had the time to weld the underplate in.
At first I drawed the lines were I cut the other piece of underplate out. I've done it this way so I wouldn't change the geometry of the front controlarms.
Then I brushed the inside of the chassishead clean with the grinder, sandbalsted some pieces and cut out the "tubes" for mounting the "stabilisatorstang".
http://www.keversite.nl/fotoalbum/fotos/22474.jpg