Pete you're my new best friend, lol. I have the same car. Only two models came with that grill trim, AAR and Gran Coupe. I can tell its a 383 by the lack of an ID pad cast in the block opposite the distributor. Mine also started out as a 383 Gran Coupe but now has a 440 transplanted from a '70 Charger. I did the same as you want to and made it into a Cuda clone until I saw the light. There is a Cuda on every corner but you hardly ever see a Gran Coupe. So I changed it back. If my two cents means anything, leave it the Gran Coupe. But do put the 440 in it!