The 6v LEDs I've found are little brighter than the incandescent bulbs they replace. There's a way to get 12v light out of your housing using available 6v bulbs. I submitted a tech tip to Ned that he's vetting now and should be up for review shortly. It's actually pretty simple. It involves putting a slight bend in the bulb bracket so you can run a #81 bulb for the tail light and an #1129 for the brake. The need for the bend is because the #1129 is much larger and won't clear the housing in the upper socket or the lens in the lower. The #81 is the same size as the stock #63 but twice the candle power.
#s 63 & 81 are on the left, #1129 second from right.

A direct side by side comparison of my 6v on left my modern 12v truck on the right.

Keep in mind that my modern truck has a mirrored reflector and my '38 6v is a much smaller housing and does not have a mirrored reflector.

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