168's and 194's interchange. 168 is brighter than the 194. Brighter incadescents are more likely (or will) melt the housing. GM used both in the clusters depending on which position.
These clusters are hard to make really easy to see simply because of the design. LED's done right will be brighter, but really making the display easier to read/brighter is going to take some effort, as you'd need to move the lights to rely less on light dispersion and more on focusing it where you need it. Doubling the number of lights would likely fix most of your issues.
For instance, even having completely re-done my cluster (adding LED's, etc), the trip odometer is still impossible to read in the dark. The stock bulb positions just aren't in the right places to illuminate each gauge face fully.
