If you want to go the Bondo route to fill in the hole, the original look still is the best. You can get the letters at http://www.mopartsracing.com/parts/emblems.html . They also have script Challenger badges and proper R/T insignia.
The guys over at billettechnology.com make custom replacement badges for the hood too. Check out video number 1. At the end he replaces the Chargers badge.