Dim Bulb Question

Can I change the bulbs behind the typical A‑body dash panel (mine’s a ’69 Barracuda) to obtain brighter gauges at night? How many watts are the original bulbs (GE158) and what can I use to replace them without melting the surrounding plastic?

The short answer is: yes. But first, let’s take a look at the dynamics of the lighting system. Before you do any swapping or modifications, I want you to know that, generally, these cars had plenty of I.P. lighting brightness—so much so that I usually drove with them dimmed at least 30 to 50%. The idea is that the apparent brightness of the instruments was supposed to approximate the light level of what you are seeing through the windshield (the road ahead), so your eye’s iris’ would not have to continually readjust as you glanced back and forth. This, to reduce eyestrain.
First, I’d be sure that the existing system is up to snuff: Clean, fresh bulbs, and check that the potential of the bulbs, with the dimmer rheostat set to maximum brightness (i.e., fully CCW just short of turning on the cabin lamps), is no more than 500mV lower than battery voltage. If low, suspect loose ground screws or a bad dimmer rheostat.
If that checks out OK, I’d try a fresh set of US or Hungarian made #194 bulbs. I’d avoid the Chinese brands. The #194 is just slightly brighter than the 158s, they draw 270 mA, as opposed to the 158’s 240mA. This is almost a universal swap.
If you still need more brightness, the step‑up is the #168 (or long‑life 168XL), which draw 350 mA, and are approximately 50% brighter than 158s. I doubt they will get hot enough to melt any plastic, but the added current draw will, if all bulbs are replaced, probably cook the dimmer rheostat, if you wind up keeping them dimmed for any appreciable length of time.