Hi Andrewh,

Ran across this company(HITECH)that makes an LED conversion kit for your 65 Coronet, not cheap at $180. It uses the same bayonet lamp mount you now have. Ask them if you'll need a "load equalizer" so your flasher will be able to blink the LED's.
Each assembly has 87 LEDs, guess you would have to seperate the lense from the housing to install this...

http://www.hitechledproducts.com/product...7cf914971c13983

HITECH's cheaper LED bulb conversions:
http://www.hitechledproducts.com/index.php?cPath=26_30&osCsid=33812d1a552415f707cf914971c13983

You might look at this company as well:
http://www.superbrightleds.com/tail-brake-turn.html

Hope this helps,
John