I've seen some pretty cool '70 Challenger convertible diecasts, however I am pretty sure they were custom made very limited edition ones, and they weren't cheap (over $100).
The old saying about if you want something done, you have to do it yourself holds true, I built a 1:25 scale model of my Challenger convertible, and my model will probably be the only FK5 Challenger convertible with a flat hood scale replica ever made.