Look in the FAQ, I wrote one up that covers everything I could think of at the time.
Don't even try to justify it on a cost basis; you're looking at a lot of miles before the MPG improvements show up as a fatter wallet after taking the hit that a 700R4 is. Do it for the lower cruise RPMs and shorter first.
You will gain in MPG; the 700R4 is what pushed my truck from a 10mpg wonder (TH400) to a 14mpg wonder. I have clocked it as high as 17 if I drive reeeeeeally carefully.
Scale of 1-10, I'd say a 4...it's really not too far beyond a bolt-up, but there are a few technical bits (namely, TV cable and TCC wiring).
Just FYI...your new cruise RPMs at a given speed will be, at most, 74% of what they are now, and that's if you either didn't put a locking converter in the 700 or if your TH350 has a locking converter. That's one of the other reasons I like it, is the 4th gear lockup.