Well, I don’t know if it’s the same language in the P40 rulebook, but the AA50 rulebook on page 14 says “If you want to make any amphibious assaults during the Conduct Combat phase, you announce your intent to do so during this phase. An amphibious assault takes place when you attack a coastal territory from a non-hostile sea zone by offloading land units from transports into that target territory…. … During the Conduct Combat phase, you can only launch amphibious assaults that you announced during this phase.”
I don’t know of any time in any A&A game where you can decide exactly what you are going to attack after dice are rolled. I know the rules quoted above leave a bit of wiggle room, but in A&A all combat moves must be exactly and specifically spelled out before the conduct combat phase. There is no such thing as rolling any dice, and then being able to decide which land territory you’re going to attack. All assignments of ground forces have to be specifically made before rolling any dice, and nothing can be changed once you start rolling any dice whatsoever, if you strictly follow the rules.
No I don’t have a direct quote or something that says “you cannot decide how you will allocate your ground units after the combat movement phase is complete” or some such thing, but I am 99.9% Krieghund will confirm that yes, the USA must announce which territory they intend to assault, and not only that, but with exactly which ground units are going to which territories. Make that 100% sure.