Plumb My Wiring

I am trying to hook up a Holley fuel pump
(electric) to my Charger (’70) and I have
plumbed it right, but their wiring instructions
are interesting but not too useful. I want to
hook it up to a power source which runs only
when the engine is running. I was thinking
to connect the power wire to the dark blue
wire with the white tracer which starts at the
alternator and goes through the bulkhead
connector at the plug labeled “N.”
If I do it their way I have to splice into a
perfectly new wiring harness which cost me
$500+ from Year One, not a pleasant option.
Is that wire only hot when the engine is
running? Will my way work?

Use a relay rated at least 30 Amperes.
Connect the relay’s input (power) to the
ammeter positive, or, even better, right to
the alternator output stud, thru a 20A fuse.
Connect the other side of the relay’s SPST
N.O. contact to fuel pump. Connect the
relay’s coil to ign‑on +12v at the dark blue
wire at the ignition switch. All wiring can be
inside. Do not hook up without a good relay.
If desired, the low side of the relay coil can
be grounded thru a toggle switch. Switched
off, this can be useful for draining the carb
bowls for storage, etc.