You might have a look at the New Series Fountain pen:
http://www.azsil.com/library/uploads/New Series Screwcap Instructions.pdf
The instructions say to use a V size drill, which is 0.377" diameter.
I have always found that a 3/8" = 0.375" diameter works just fine.
Both tubes of the New Series are the same diameter. I will get you the lengths in a minute.
EDIT: Here are the lengths:
The New Series tubes are 2.09" and 1.82" in length.
Compare this with the Cigar tubes which are 2.09" and 1.91"
I don't think you would have any trouble using the 3/8" tubed Circuit Board blanks on a New Series kit as far as tube length goes.
The 3/8" C.B. blanks that BearToothWoods sells are from PSI.
The Cigar size C.B. blanks at BearToothWoods are tubed with "10 mm" brass tubes which are 6 or 7 thousandths bigger than the 3/8
I prefer the Cigar blanks as more authentic circuit boards but they would be 6 or 7 thou too big for the New Series pen components.
That difference in size could be made up by a brass ship about 3 or 4 thou thick, but that would take a tiny bit of fooling around and Rebecca may not want to do that.
You can buy both the New Series kit and the 3/8 C.B. blank from BearToothWoods in Colorado Springs.
You can find the specs for both Cigar and New Series kits at the Berea website or here on IAP website at this location:
http://content.penturners.org/library/general_reference/berea_bush_tube.pdf