I found on ebay a 6V bulb that promises to be brighter seems to be true. A #17301 bulb 6V 10W incandescent bulb is much brighter than my traditional #63.
I put them in my 1935 single sided taillight and they really are. They were only $3.75 each. (Much cheaper than other options). They say the original application is medical equipment. Time will tell if they last. I wanted to share because, "I saw the light!"

Ebay 161410757417

You may have seen the light but were you blinded by it????

Bright is nice as long as there is not a big increase in heat build up. Especially if using plastic lenses.

I would use a #81 for the tail light. It's much brighter than the #63. It's 6.63 watts & just over 1 amp. Use the 17301 for your brake light. Painting the inside of your housing gloss white also helps get more light out. Below is a comparison of my 6v '38 using a #81 as the tail light on the left & my modern 12v pickup on the right.

Ther "brightness" of the bulbs was rated by candle power the #63 bulb was 3 C.P, an 81 was 6 and a 87 was 15 as an example.

These are said to be 7 cp. 10w
Typo, thanks M006840

The listing on Ebay has them at 7 candle power which is a little more than double the #63 bulb.

FWIW I use the #1129 for the brake light. It's 21cp. It's a larger bulb so I have to slightly bend the bracket to make it fit. I don't know if there's enough room in the '35 housing to do that.