You don't stand a chance of making it. Sorry. The reason is relatively simple.

The lines at SEATAC customs and immigration are horrid on a good day. The ONLY way you'd stand a chance at making it would be if you flew F or front rows of Biz. Had ONLY carry on luggage. Lastly, because you'll have a 99% chance that it won't be in the S terminal, you have to:

1) Clear security whether you are leaving the S terminal or staying in it (that is to be fixed at some point according to a post on this board).

2) Clear security again at a different terminal. So, your doing a double clear of security along with passport and customs, and the train running every 7 minutes.

I'd just get yourself covered or moved to the next flight.