If you're eventually going to a built 351W, I would not use the T5. It'll break soon after because there is a lot of torque being produced by an engine like that. I would either go with a Toploader 4-spd or a C4. C4s are light and fairly durable, with only a few modifications they can be awesome. Get a good shift kit, a good stall converter, new filter, and new fluid and they work great! Very efficient. The AODs are heavier and can't be shifted manually like the C4 can.
Hooker Super Comps are probably about the best header for a street car. But they are VERY expensive. If you have the money, buy them, if not, just get a set of Hedman Hedders. They are right around $100 for full-length headers.
You will definitely want to get new shocks all around, of course, KYB Gas-a-justs are a good choice and are fairly cheap. New leaf springs are a must also. I'd get a heavy duty pair for sure.
As for the front suspension, you'll need to check out what kind of shape everything is in. For sure you should get new polyurethane bushings. If you're going to get into handling and autocross you will need big sway bars, front and back, and you should lower it like
Clark Rodgers suggested. Also do all the ball joint replacements and other stuff
Clark mentioned.
If the car is going to just be a weekend street cruiser, I would recommend not going with the lowering kit. If you're going to drag race it and not autocross, I would recommend not going with the lowering kit. You'll need the extra clearance for big tires. And you'll also need extra ground clearance for when the front end comes down from the sky and bounces after launch.
I would go with a 9" rear. Gears depend on what tranny/cam you go with.
NOS stands for Nice Old Stuff I believe.
bs