Greetings! I'm working on a vintage car that had (when I disassembled it many years ago) an adaptor to mate a vacuum hard line to the intake manifold. After searching too long, I'm going to declare the adaptor lost and try to make a replacement. The hard line is 3/8" steel, double flared and fitted with a copper-tubing-style compression ferrule. The flare nut is 9/16-24. The intake manifold is tapped 1/8-27 NPTF.

I know how to make the 1/8-27 male end of the adaptor, and also the female 9/16-24 thread. I'm less clear on how to make the angled seat for the ferrule. I believe the included angle between the tapered faces is 140 degrees. That's pretty close to the standard drill point of 135 degrees. I see two ways to go about it: 1) re-grind a drill to the desired angle and reduce the rake of the leading edge so it will leave a smooth surface, or 2) make a D-bit with the desired angle.

Any suggestions?