Hi oldblu, I got my l.e.d. conversions from http://www.truckandcarshop.com/trucks47_59/t47_081.html
These fit in like ordinary lenses but you have to remove the original bulb holders.
You may have to replace the flasher relay to allow your turn signal lights to flash at the proper rate.
I'm not sure how yours are wired as I will be using my bed roll lights(Amber) as turn signals (UK requirement) and my normal lights as tail/brake lights. I will be adding a third brake in the rear window. I've tried my tail-lights across a battery and they are really bright!

1950 Chevy Advance Design 3100 in Scotland
In the Stovebolt Gallery
More pix on Flickr.
I've definately got this truck thing in my blood ... my DNA sequence has torque settings
"Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.