I'm currently wiring my dash, so I thought I would post the information I have about the repro low fuel warning module that I have..
These are the supplied instructions and it shows that the install is really easy.
In my console, I have 4 Autometer gauges, one being an isolated fuel pressure gauge and I use a NO (normally open) grounded pressure switch to illuminate the "FUEL" lamp when fuel pressure drops below ~3.5psi.
I have fitted a 6X8K factory tach so I have to have the fuel level gauge in the center of the instrument carrier with the module sharing the low fuel lamp. I know I don't really need the low fuel warning lamp with the gauge right in front of me, but that the Hell. This is fun!
I also fitted the module with a 4 terminal connector (Packard 56 Series of course!) so it's a serviceable item if it ever fails to work.