Hey Friend,
When you speak of a clearance of .0015", than is the clearance I hone an exhaust valve guide to in relation to the valve stem dia.
There is a perceptible wobble of the valve when inserted into the guide. I don't think that is what you want. Using anything other than oil when pressing in a valve guide is frowned upon by "Experts" because it will hinder heat transfer from the guide into the head for dissipation.
If you use Loctite on a press fit, you better be pressing it together fast, because even a pause of a few seconds can allow the anaerobic to start and it will seize up.
Drill bits wander, even drilling an existing hole, especially over a distance of several inches. Forget about drilling from both ends, meeting in the middle, DOH !
Look at a "Piloted Core Drill Bit";
http://pacifictoolandgauge.com/33-pi...-drills-nopix-
The above is only an example (the 1st image I could access). It will faithfully follow the existing bore if you can get a proper sized pilot tip. There are some that will have a 90 degree cutting edge that would allow you to leave a square shelf as a stop at the muzzle end if you halted .060-.090" shy of breaking through. The ID of the hole left by the core drill determines the OD of your new barrel blank. Drill 1st, then size the barrel blank accordingly.
The Silver Solder is a good idea, because it would not be as notable as a step or a pin.