66 dart LED prototype working for now

Your meter readings give cause for concern; the bright/dim intensity ratios aren't anywhere near sufficient. Only a 49% difference axially for the original bulb, and only a 30% difference axially with the LEDs. I'd be inclined to try to rule out a problem with your measuring method/equipment as a first step, because the original lamps did comply with the 5:1 and 3:1 ratios. You may need to tweak your driver circuit to reduce the intensity in dim (tail) mode.