Hi Mike and Mike :-)
Like 35Mike, I too believe your truck came with only the left tail light.
By both of my Chev and Maple Leaf truck parts books (1938-1944 and 1938-1946) the same tail light housing was used from 1940 up to at least 1946.
New reproduction housings have a slightly different base the sockets mount to and may have more room for larger bulbs than the originals.
That said...
By my research, BOTH the tail light bulb AND the brake light bulbs were originally equipped as #63's, which are 3 candle power.
I believe #67's are 12 volt, so you won't be using those unless you've converted your system to 12 volts.
You should be able to upgrade to #81's, which are 6 candle power.
If you have an original tail light housing like I do you WILL NOT have room to upgrade your brake light bulb to a #1129 which is 21 candle power, or even to a #87 which is 15 candle power, as it is the upper light in the housing and too close to the glass lens.
You COULD upgrade your tail light bulb to a #87 by bending the socket mount in a little as I tried in the picture I've attached.
The problem with that was that the #87 tail light swamped the #81 in the brake light.
For that reason I settled on #81's in both sockets as I don't drive much at night, and usually under city lights anyway.
When I get around to it, I'll clean up the inside of my housing and make it more reflective as well.
Hope that helps.