My one experience flying through GRU: Customs took less than 5 minutes. Late afternoon flight from Lima Peru. If you are an American the line will be shorter for you, as long as there are more Brazilians than Americans on your flight. What took the longest was getting my bag, that was maybe 30 minutes.

Personally I think you could make it but I would be more concerned with a flight delay. Any sort of delay out of LAX and you probably won't make it. I'd schedule a longer layover if possible.