Nitrous build questions.

Are you planning on a wet or dry kit? Nozzle, plate, direct port? How big of a shot are you looking to run? How do you plan on tuning the car?

A bigger throttle body won't make a big difference with the nitrous. In theory, you want velocity for the nitrous mixture and if you're engine is capable of moving it, the bigger the better. On an otherwise stock, or near stock 4.6, that's not really a concern. That's also assuming you're using a nozzle kit. For a plate or direct port kit, the effect is lessened. That being said, it won't hurt to get a 70 or 75mm. I have a 75mm on my car. No big gains, but throttle response increased a little and it may feel a little stronger on the top end when coupled with the ported plenum.

I'd skip the underdrive pullies and save the cash. Steeda makes a good set for our cars that actaully uses an SFI approved dampner as compared to many of the cheaper piggieback pullies... which have caused a lot of debate. A lot of pretty knowledgeable people claim the piggybacks can throw off the internal balance of the engine and shatter the oil pump. N/A they make a small difference... but $180+ worth? That's your call.

Intake manifolds.. Edelbrock's intake is SICK for the 4.6. It's less expensive initially, although it does add up if you prefer not to fab small parts to make it work without buying more parts for it. It's a bad ass nitrous intake though. Short, straighter runners and its made of metal. No nasty nearly 180* change of direction like with the stock and many aftermaket intakes. However, you either have to adapt the stock plenum to the intake (which chokes it down), or choose from a couple of plenum options... none of which are cheap.

The stock PI intake WILL work fine for spraying unless you have the unfortunate nitrous backfire, then you can plan on it shattering into 100's of pieces and needing a new intake. I'm spraying a 125 shot through a stock PI intake and so far so good. If it popped, I'd get the Edelbrock intake.

I've heard people really like the P51 intake aside from the fact it's A) heavy and B) holds a lot of heat being all metal. I wouldn't personally use a Pro Products intake because it's too close of a design to the PI intake and while it can be ported, it's also heavy and heat soaks terribly. It still has the long runner design with that sharp bend the PI intake has.

All that sounds good. Subframe connectors are a great choice. Go will full length weld in's though.

NX makes great stuff. Do you want a nozzle or plate kit? Plate is superior and easier to install, costs a little more though.

12's on street tires should be a breeze as long as you're getting some type of traction. If you still have stock gears that will help.