That'd be fine as well. Ni naru and to naru are both grammatical and both are used, I've heard both quite often. Ni naru is used when becoming something is a natural process and kinda just happens on its own due to external circumstances, while to naru signals agency - a definite intention to become something. To naru in this case would reflect Light's intentions better - he doesn't just become a pimp-ass god by chance, that's his goal from his very beginning, that's the thing he's actively working towards.
(I'm not actually that smart, my source was here:
http://okwave.jp/qa/q4499016.html )