if you're writing a song that's message is through the words, you want the most flexability in expressing them, and by writing them with the melody first, you have the freedom to take the melody anywhere you want it to go. there's no need for getting stuck with only a couple of melodic options just because thats how the music goes. as far as the sylables timing and rhythm go, just keep the timing with a metronome, or even your foot, and you'll get the feel for the rhythm of the words. :)
Dont take life too seriously... you'll never get out alive :)
