http://en.wikipedia.org/wiki/Porsche_935
I quick read through did not suggest any comments about changing the rake of the screen and so on though no doubt the rulaes for certain series or events at certain times may have permitted such a change.
However it it is clear that the targets (of the race series' and the Porsche engineers and the independent teams using the customer cars) were constantly moving - which observations thus complement JR's comments.