Although I have 3.73s myself, I would suggest 3.55s or 3.31s unless you are wanting to take your car to the track. The track is what I'm getting into, so I'm very happy with my 3.73s. I also love the hybrid tune on 93 octane -- but only during autocross season. On the street, mpg is far inferior to stock 2.73 manual. I was getting up to 32 mpg on the highway with conservative driving, no tune and 2.73s. Driving the same way my best on the highway is 26 mpg. On the interstate at 70-75 mpg I lose 1-2 mpg. I'm okay with this because that's the sacrifice you make for weekend fun.
Weather permitting. I also lose 1-2 mpg while on the hybrid tune. My mustang is not my long trip car.
However, if you don't plan on taking your car to the track, I would not suggest 3.73s for you. 3.55s won't make as big a dent in your mpg but will still be quite spirited on the street. 3.31s are close to the same and have the advantage that you will not necessarily have to spring for a tune. And they are still nicely spirited, a good step up from 2.73s. Also Ford can calibrate your car's computer to account for 3.31s, but they do not provide a program for any gears deeper than 3.31s.
As for a one-piece drive shaft, yes I have one. I don't regret it one bit, but of course I expect my car to be on the track come spring again on Driving Experience days sponsored by my Mustang Club as well as autocross. I believe in safety first and since my DS will be rotating much higher with my 3.73s than with the stock 2.73s, it simply made sense to me that I should replace it. HOWEVER, if you aren't going to track it, you don't need the one-piece. Save your money for even more interesting mods if you like. Don't plan to do high speed driving on roads -- I'd like to think you want to keep your license so you can continue to enjoy your mustang.
I suggest all these things above because I want to help a fellow enthusiast enjoy their car. We all drive differently, so please consider your use first. That way hopefully you won't find yourself disappointed later.