That looks like a Hobbs switch. I don't know where it would hook to a t-bird/cougar motor, but on my GTP I have mine hooked to a boost line (same place I'd tie in a boost gauge). It closes the circut when you reach a certain ammount of boost and sprays the giggle gas, at least that's how it works in my car.
Oh, and don't buy a bottle from NOS or NX, they're way over priced go to this web site and get you bottle there. http://www.apisureshot.com/order.htm.
You can get a 10 lbs bottle for $85 and a 20 lbs bottle for $110. I don't remember the other costs for the or the 15 lbs bottle, but they're between those somewhere. Mine will be in the mail shortly. If you don't go thrugh them still be sure to get an aluminum bottle. I had a 10lbs steel bottle and I couldn't keep it hot enough to keep the bottle pressure above 700 psi, even with a zex heater. My buddy has an Al bottle and, on the same day I couldn't get mine over 600psi, had a bag of ice on his to lower the pressure instead of fighting to raise it like I was. You want to be at about 900-1000 to get the most out of it. At 600 or so you're really only seeing about 1/2 of you shot.
If you have the solonoids, line, nozzles and switch you're really only lacking a bottle. I don't know what's involved in a purge kit, but you said that's all there. You will need a pressure gauge to go with the bottle, but I wouldn't buy one from a n2o company. I got mine with the "T" fitting for $16 from a place that sells co2 tank supplies. Pressure is the same no matter what is pushing it, so any gauge that goes up to 2-3000 psi will be just fine (you should never see pressure that high though).
I wouldn't know where to go about installing the nozzle on a 4.6. I would assume at the TB though. I have to spray after my TB, straight into my blower through a TB spacer on my GTP cause the MAF is part of the TB. I think you can spray at the end of your intake tube right before the TB unless I'm forgetting something about that motor.