These days I do have trouble hitting the same Cs myself - but then I'm a tenor. The best advice is always relax, think high, and just hit it. Smiling will help to raise you voice just a little bit - seriously. Enjoy the Mozart - it's a great work - even if some of the music was pinched from Handel !

The piece of choral music which I reckon is the real killer for First Tenors is the Lachrymosa from Berlioz's Requiem. Usually it's best if the singers take turns on who pushes it in each of the repeats. Mind you, later on in the same work during the Tuba Mirum, The Basses (and usually the Tenors as well) have to sing against the biggest wall of sound (huge orchestra + 4 Brass Ensembles + 12 sets of Tymps) that I've ever heard in classical music. You can never succeed in getting over the top - Berlioz was trying to show that nothing could outdo the last trumpet !