Do a
swagelok compression union and be done with it. No tape needed, no stupid seals....just quality stuff!
Funny, I had this discussion with them just yesterday on another board. This is exactly why I don't have meth yet....too much crap out there and hard to get quality stuff.
Edit, if it's 1/4" OD you're using,
here is one on ebay for $8 shipped. If you want to get really fancy...do a bulkhead fitting at the firewall or where ever you transition.