As zpaul has said, this is quite risky. Possible, but risky.

If you happen to be on a single/linked ticket, I would risk it. There are other USAir flights (the 6:30 flight is USAir not United I think) at 8:30 and 10 and if you are on a high fare class on linked tickets, they will take care of you.

If this is constructed from cheap unlinked tickets, I would explore how much it would cost to shift to the later flight now. You probably don't want to be risking it arguing with USAir why you missed your nonrefundable flight and if/how you should be reaccomodated and what fare differential you should pay.

If you do stick with the early flight, try to get a seat at the front of your cabin, and walk at a fast pace to get to the head of the immigration line. At this time of the morning, if you beat most of the plane to the line, you should be through quite quickly. However, if you get stuck in the middle or end of the line, staffing levels may be inadequate (esp. if some other south american flights arrive at the same time)

Good luck!