Not sure of what type of tolerances you can achieve in your shop, but the tolerance of most pen kits really aren't that close. I would measure kit parts then bore/drill, just make sure you go a little on the "narrow" side, can always sand out the extra so the press is good snug fit.
Thinking outloud; if you TBC/chuck you can drill your pilot for the tip section a little over the diameter of the refill and drill out what is needed for the tip/transmission to seat, likewise on the cap to accommodate the transmission/Cap. Should help beef up the pen and give it some weight and durability. Would definitely have to tinker with it to get a good balance, but might be worth it in the end.
