I had very good luck using SEM plastic bumper paint. I used in on the plastic shields for the fenders/doors (E-body). They turned out like brand new, and do not look painted at all.
I first glass beaded the parts using a very fine glass bead, then gave them 3 coats. This should work on your rad shroud too.
I also tested this on a junk part first, and the adhesion is very good. I flexed it back and forth many times, and no problems.