Made the trip several times since i have lived in Skagway. When the weather is good, by far my favorite Alaska flights.Yakutat has fuel and the family that bought the WW2 era Hanger is in the process of restoring it and are hoping to create a museum, i enjoy looking at the progress each trip.
Love flying over the glacier fields.
On customs, the only thing to keep in mind is that most of the airfields in SE AK are considered "at will" (including Haines and Skagway), meaning since they are not manned, customs requires advance notice and if they are understaffed, they may not allow you to land there as they may not have someone available to run down to the field to clear you. Doesn't happen often, but as we head into the tourist season, they get busy and some are getting pulled for assignment down south, they run short. Kind of like i was saying about calling to confirm fuel, way better to know in advance.
Hope it works out for you, it is a flight you won't forget. Sometimes i will take the longer route, fly up the coast after exiting the Tatshenshini River and Alsek Rivers, as gbflyer said, stop in Yakutat for fuel, (You can never have to much fuel out here), look over the hanger progress, maybe eat lunch at the lodge on the field, fly over the glacier fields ( with a quick tour of Yakutat Bay) to just south of Cordova then follow the Copper river up to Gulkana.
If you end up in Skagway, let me know
Chris