Ernie Sparks

I have exactly the same set up on my '73 F250 XLT with the 390 engine.
However, the bolt hole on my exhaust manifold is virtually perpendicular to
the flat surface of the horizontal boss. Do you still have the old manifold?
If so that should give you a real good indication of where the hole should
go. If not simply bolt the bracket to the back of the A/C compressor and see
just where the hole lines up on the exhaust manifold. I'd simply punch a
mark, drill down a half inch or so (no more) with a 1/8" to 3/16" bit as a
starter hole. Remember, you're drilling cast so it will drill differently
than standard mild steel. The drill tip needs to be dressed much flatter
than that used for steel. It seems to me that the hole is tapped 1/2-13.
This can be a problem if you don't have a "bottoming" tap. I have, over the
years, gradually made "bottoming" taps by slowly grinding down a standard
tap. If you do this be sure to take your time doing it since you can change
the metal characteristics dramatically if you get the tap too hot. Secret:
grind a little then cool quickly in oil. Grind some more, etc. Try not to
let the metal get hotter than you can stand by holding it about a half inch
from the area you are grinding on the tap. If it gets hot, stick it in the
oil.
Good luck.