LEDs are the way to go.
And, as you are taking everything apart, you may wanna paint the inside of the instrument cluster in satin (flat) white. The plastic turns yellow with age and does not reflect the light as well as it is supposed to.
Be sure to use satin white as bright white paint somehow does not reflect light as well as satin white, for what ever reason.
While you're at it: The black rings that you find around the faces of all three gauges should have their backs painted white too.
The reason is, they wrap around the side of the faces and absorb some of the light that comes from behind the gauges.
That way you are not only using brighter light sources but you get maximum gain from them.
This is what the white LEDs look like with the old stock blue reflector caps :
By the way, on the picture you can clearly see the reflection from the white backs of the rings on the faces of the gauges. That really makes a huge difference!
The green tint we're all used to see is due to the interaction of the yellowish light emitted by the standard bulbs and the blue filters.
These are the normal LEDs from Mustang Project.
Although these are a great improvement, I would use the extra bright ones, if I was to do it again.
That way I could dim the lights a little and have a little "extra" if need be.
I still have the regulator turned all the way up.
As somebody posted above, they are a standard replacement. All you need to make sure, is that they have to be inserted the right way. Otherwise they will not work. I took the trial and error way and it worked fine.
Anyway, they are so bright that you can clearly see the blue light even in a lit up underground parking, which is a great improvement!