What's that middle one doing G? Separate turn signal?
Seems like having that one come on too, would give you some more "fill" in the lens and reflector. Rather than the central spots that show up in the images.
And speaking of that, is the entire lens lighting up, or is it just the spot right at the bulb (like normal for that type)?
One thing that definitely helped in my case, both with incandescent and LED's, was painting the reflector a bright white. I used an "appliance white" spray that worked real well. Just make sure it's glossy, rather than the matte finish that we see occasionally.
I tried aluminum paint, reflective tape, standard white, silver, and just bare housing with a slight polish to it. The gloss white outperformed them all in my tests.
Never did get enough fill with the LED's to make me happy though. Haven't tried the newer types with LED elements facing in all directions though. Nor did I get around to installing the LED strips and different board designs I was playing with.
Just ran out of time with family matters taking precedence.
Good luck. I like the idea of another red LED in the backup light socket. Have you thought of buying the '66-only tail light lenses? Those don't have the clear section over the backup light, so would show more consistently than your '67-'77 style.
Just a thought.
Paul