Congrats on the shelby, it's a very fun car and loves to be modded.
If you want to pimp it like your FGT, it will take 22's out back, but I strongly suggest staying away from them. Once you start adding power and changing the suspension to handle better, it will be hard to get traction on such a short sidewall.
There really isn't an optimal PS2 tire size for 19's or 20's for the shelby (they are all too short or too narrow). If you go 20's out back I'd go with the 285/30/20 ps2.
I see that you're going with the full stainless works setup, like me. I haven't installed them yet, but it all looks awesome. (hopefully getting installed next wednesday) I went with 1 7/8" headers, a 3" catted X pipe and their 3" catback. They offer different mufflers so be sure to specify what sound you desire.
As far as power, right now the 2.8L KB kit is all we've got to choose from. Whipple has a 3.3 prototype running around on my buddy JD's car down in San Diego. He recently made over 700rwhp with a tune from Greg at Racers Edge Tuning. If you want to push power further, i'd see if MMP has a FGT block they'd like to sell you and have boss 330 or accufab build you a nice short block.
Re: suspension. Right now the best you're going to get is griggs. They completly change the suspension to different set ups...the biggest down fall is that it's very pricy. Expect to spend 10-15K + install. They are also in california, so you may be able to work something out with them, as I know you are very resourceful. I hear they are anxious to get their products on cars, so I am sure there are discounts to be had.
If you decide against going crazy with them, I have the full ground control coil over setup, and quite a few BMR rear suspension parts. Either way you decide, I'd probably go with the full griggs rear end (as if is much better than what I have) and isn't nearly the pocketbook smack that the front end is. PM me if you'd like some help putting together specific things for your ride.
Oh, and for the time being, do yourself a favor and get a tune...it really makes the car much more fun since the stock tune sucks ass.