You know what? I never really saw the cold cathode part before...
I think that might actually work fairly well to light up the cluster the best since they throw light off in all directions like the incandescent bulbs, and it's a higher color temp, so would look a bit different.
But for the brightest, I was looking at that dude's site who sold us those janky guage overlays, and there was a good idea from a thunderbird on there to maximize the light. Make the back part of the cluster reflective... easiest way is to paint it like the highest gloss white you can find (or other light colors, silver was on his page) to reflect as much light as possible.
Or maybe try some aluminum foil to reflect the light. I dunno...
I just stuck some 180° 1 watt white LEDs in there, and it made it pretty damn bright. But those things are like $5 a piece, so it's not so much of a cheap mod.
But yeah, make sure to take some pics if you do the cold cathode method (and btw, I found cold cathode tubes for free in old scanners that were 12V

)