75 Dart Instrument Cluster

The diffusers keep the bulbs from falling into the housing. I have a similar 73 model panel here. It has flat pieces of blue tint film instead of dome shape diffuser. serve same purposes.
The white or light blue paint inside was to reflect the little bit of light they had. Looks like you're going to need something brighter than OEM bulbs.
As for that white plastic piece, don't know. My 73 panel doesn't have that or the rectangular opening you have below temp gauge.