You mean you can run a resistor in line to the LED's to lower the voltage(and light output)?
Yes, the great thing with LED's are that they need no more than a specific voltage to work. You can "overdrive" them, but they will burn out quicker or instantaneously.
If you get a 12v led, you can use a resistor to lower the input voltage so the bulb is dimmer down to 1 volt. You can add a variable resistor so you can actually adjust it using a dial if neccesary so you can "tune in" the look you want.
As long as you know how to solder and can add and subtract, you can wire LED's. If you reverse the polarity, nothing happens, so switch it around again and bingo you're good to go.
BTW, the inside dims width wise for a 72 up Challenger is just a bit over 6" wide.