Back in the day on a previous vehicle I had this cluster configuration as well. Mine had green caps that went over the bulbs. I wanted to change it up so I pulled the caps off and switched the bulbs to red coated bulbs. Even so, reading the odometer wasn't a strong suit in this application, it always seemed washed out. I also kicked around the idea of going for UV lights for the dash lighting, and touching up the cluster with florescent paint. Never got around to it though.
Incandescent bulbs also dim over time, so I wonder if new bulbs alone may address the problem.