Inside is per socket and bulb links already provided.
Directly. Remove/replace, no mods needed.
Exterior is trickier, because you're dealing with safety lights that have to work right (not just light up), and there are optical complications. Start reading at post № 80 of this thread. That situation is in flux; one of the world's reputable light source makers (Tungsram, long affiliated with GE) is gearing up to launch a new line of LED signal bulbs that stand a good chance of working well in old-car lights. As soon as they're released and I've tested them, if they're as good as it looks like they might be, I'll shout about it on here.
Headlamps also require very choosy and informed shopping; see here.
I haven't added anything extra to mine and I am slap happy with it. All I did was add the two super bright Zevo LEDs to the two dash lights and that made them very bright. Thanks @slantsixdan for all the advice.
This is just from swapping TWO sockets to the 168/194 type and installing only TWO led bulbs. That's ALL I did. Why you need to do anything else, I don't know.
This picture is actually with the first LEDs installed which were yellow. The camera picked up glare which was not there. I decided I didn't like the yellow and changed them to white. The white looks much better. I'll try to remember to post a picture tonight.
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It's very interesting to me that the barracuda had only 2 illumination lights in 64. the dart of the same year has a similar layout and size, and has 7 total lights in the dash. 3 for illumination, one for oil, one for high beams, and two for turn signals