Really like the work on the Merc so far Dave, its gonna be sweet! Like Tech inspector and streets have said, a continuing theme is a good way to make sure the modifications to the grille dont look out of place. Remember that perspective is important also- even down to the distance between the slats in the grille ( if you choose to have that design) tally with other gaps ie- the gap between the fender and the body for example. Ive always thaught that this sort of attention to detail can make all the difference.
Just a thaught: have you considered copying the design of the origional grille but exaggerating the contours? Like where it comes to a little peak in the centre to match the bonnet (im assuming thats what it does- not as familiar with them) it could be done this sorta shape ----> {
Assuming your running a front bumper (fender?) this could be modified to match also?
Not sure if thats any use to ya, but if your anything like me, any imput is useful when your a bit stuck for ideas. Good luck.