Hi Jerry - the 47 heater box is ALL metal! It looks to have been painted some kind of light to medium brown or brown hammertone.
After a lot of thought the 47 is getting the 70 340 that is ready to be rebuilt mated to a Dakota A500 that Dynamic Converters rebuilt as well as reworking the torque converter for increased stall. 8.75 Rear end is from a 71 Charger which should be a virtual drop in according to the info on the P15/D24 forum. With 3.55 sure grip, acceleration should be good and with the A500’s overdrive, get reasonable decent economy. Still a ways to go on this project - gotta get the 65 finished before tackling the hard stuff on the 47!