If you booked on line you should be able to reserve seats online ( I could with delta) . If not (such as me this time) I had to go through the booking agent (thomas cook) and they sorted it. I was restricted to six weeks before. Also they said they couldn't book the connecting flights as United airlines only allow 20% of seats to be pre booked? Not sure and not really bothered about these as long as the long flights were booked.
Regards
Brian
P.s US airways have a good reputation and have plenty of leg room.