Has anyone adapted LEDs for dash bulbs?

It is true that many l.e.d.s cannot be dimmed. The ones that can do cost more. If you go to:

https://www.superbrightleds.com/cat...ulbs/vehicle/1972-plymouth-duster--/47-460--/

you should be able to find some that will work for you. There are different ones available to fit your car. I'd recommend trying to get some rated around 100 lumen or brighter.