A 7.3L Excursion would have come with a 6G small case alternator rated 110A.
A dependable upgrade would be a Genuine Motorcraft 6G large case alternator rated 140A. They were used on the 6.0L F450's and will fit on a 7.3L with a minor grinding to relieve the interference at the bottom of the alternator mount. Unless you have a ton of aftermarket electrical loads, a 140A alternator should serve you well.
You should let us know the specs of your truck so you can get the right advice.