Of the two plugs on the sender, one is to the guage, is the other a purple ignition wire or a ground wire?

I would think the other is the ground.

I thought so too and wired it that way and got to wondering where the juice to power the gauge came from. Guess it has an ignition wire to the gauge it self.

Yes it is the ground. The gauge has it's own power source. Typical gauge will have a neg pos and sender marking.

Thanks Guys

I'm getting ready to mount my fuel pressure sender also, I'm taking the plug out of the back of the fuel rail. It's a tight fit with the distributor cap right there (2000 502 Mag MPI) but it'll have to do. Any idea as to the thread sizes of the sender and fuel rail ?

Thanks,

Rock

I would think its 1/8 pipe thread ....

The sending units are basically variable resistors that send different levels of a ground signal to the gauges...which then shows what the reading is.
If you have two spots for wires it most likely one gets a ground wire......or....you may have a dual station unit..meaning it can feed two gauges...seeing that its going to be exposed to fuel..and most likely be using teflon to seal it up it would probably interfere with the ground signal..thus the two wires.
As for the tight fit on the fuel rail...you can always use a 90* fitting or extensions to get it out of the way...