I've recently found by Limes (Hungary) some "laser cutting" grills for GMC (fit for Heller, Academy and PST GMC), symetrical and asymetrical, two grills (with and without winch) for Esci-Italeri and Academy Ambulance and PST Studebaker...
They fit perfectly, exept for the Academy ambulance because the mud guards are not bended enough at the front, but it can be easily repaired, like this :

here is the result with the Academy cab :

and with a PST Studebaker cab and a welded grill :

It worth it, even if it's really delicate. I can say it's better to glue them at the really end of your built !!...
You can have a look here :
http://www.limesmodel.hu/webshop-b/index.php?route=product/manufacturer&manufacturer_id=11&page=1
Pascal