The stock 1938 would have two #63 bulbs in the housing, both 3 candle power. An alternative is using a #81 for the tail light and a #1129 for the brake light. A slight bend in the bracket may be necessary for the 1129 to fit. The 1129 is 21 candle power. If your tail lights have been converted to use a single, dual filament bulb with offset pins you'll need the #1154 bulb. The headlights used the #2320 prefocus base bulb, 21 candle power low/32 candle power high beam. A brighter option is the #2530 32/50 candle power. If your headlights have been converted to something else we'll need to know what you have now to advise a 6v alternative. The stock dash lights were #55 bulbs and not bright. I swapped to halogen bulbs but the supplier no longer makes them. Someone else may be able to advise you on that. The dome light stock is a #81 but I use a #87 which is twice as bright. It's slightly larger but will fit. Perhaps knowing the numbers on the bulbs you've been using can help us give you 6v alternatives. <Stock Bulbs>

@tiny Thank you very much for the detailed information. It will help me a lot.

The car is still using 12v lamps, although I have converted it back to 6v. The only lamps that I have alread changed were the rear lights. I installed 5/21w. They are working fine.

Do you have suggestions for the turn signal lights? The car uses a single socket with two filaments. Is it 5/21w too much? Could I use 5/17w? What is your recommendation? 

If they're BA15 sockets with offset pins you'll probably need 1154 bulbs. I have no clue of the wattage.

@tiny Thanks. I am wondering about the wattage that I could use in the turn signal. Probably 5/21w is too much. I will wait for otherˋs comments. If nobody makes any suggestion I will try 5/17w.

In the US we use the bulb number as the identifier. The wattage is a part of the bulb specs but, in my 76 years, I have yet to hear someone asking for a bulb by wattage. When researching bulbs, looking at the wattage is advised, as is the amperage, but when actually shopping for said bulb we use the bulb number. If you can provide the bulb numbers you are replacing with 6v, we can help identify 6v alternatives. The bulb number is normally printed or embossed on the base of the bulb. <For example you can see the number 1154 in this image.>

Very interesting your comment. In my country we identify the lamp per socket size and model (e.g., ba9s), voltage and watts. Never per bulb number. The wattage intends to show how powerfull It is (i.e., to illuminate more or less). 

When I read your responses, which had only bulb numbers, I had to "google" at the sabe time to understand what you were explaining!!!

Today, because of the introduction of eletronic and led lamps we also use lumens info.

Once again, I thank you for your very technical comments.

Unfortunately wattage isn't always linear in relation to brightness. I discovered that when I was researching brighter bulbs for my 38 and 53. Actual light output in lumens or candle power is the primary driver. After that we look at voltage and bulb structure (base size, globe size and shape). That leads to the bulb number which we use when shopping. Why do it that way? It's simple. Using the #1129 bulb as an example: Every #1129 bulb is the same physical size, has the same connection format, uses 16.8 watts and uses the same voltage. Using the wattage as a guide, not every 16.8 watt bulb is a 6v, BA15S base, 51mm long bulb. If we go into a store and ask for a 16.8 watt bulb we likely won't get the bulb we need because they are not all the same as they would be when we ask for an 1129 bulb.

Just a thought. Has the car been rewired for 12 volt at the time it was converted to 12 volt? The 6 volt lamps will be drawing a heavier current than the 12 volt lamps and necessitate larger gauge wiring.

Thanks @bob-remm. The car was "half" converted to 12v on the top of/using  6v wires. I am reverting it back to 6v. Thanks for the caring.

Another thing that can impact how bright your tail lights appear is the condition of the reflector.  If it is in bad shape I have found you may actually be better off painting it with the brightest white paint you can find than if you were to paint it with a chrome or metallic paint.  Several of the chrome and metallics that I have used have a black base that actually absorbs some of the light.  White will not absorb the light.  Best case is to have the original chrome still in good shape but some of us are not that lucky.  

High gloss white paint is the best alternative silvering. As you noticed, the "chrome" paints are shades of black.