Heater box question

[Chopbuilder]

I have a 71 duster 6cyl w/ac and I'm looking at a 74 w/ac and a V8 disc brake car to swap over running gear. The heater box is fiberglass in the pics he sent me and I don't see that on any other cars of that year. Can anyone tell me if that is wright for a 74. Thanks.

 

[RedFish]

everything about heater boxes, controllers, and wiring changed. 72 and back, 73 and up.

 

[Chopbuilder]

Thanks, I had pulled up some pics on the net and they all looked like my 71 even tho it said they where 74