Just catching up on threads while I was out of the country.. So many..
Brian is right, you can use any threads you want to make the kit less pens and the idea of running a 6mm die down will work. My dad did that many years a go when he used to make them. However, if the customer wants to replace the nib then the new ones won't fit it, been there done that. 
I have the 6.4, 6.5 and the 7.4, 8.5 and the 8.4, (all x5) luckily that's what my dad had left me..
used them last when he was alive but I must get to using them again.
We only have one tap of each and remember it being a talking point with my dad at the time, but if you take it slowly and ensure that its lubricated well you should not have any problems.
Anyway, if anyone wants to borrow the above taps for any specials then PM me and will send them out.