How much time is needed for international to domestic to international flight transfer procedures?

Hello, I have reservations on China Southern Airlines on 17 December 2012 from Kathmandu, Nepal to Guangzhou, then directly to Beijing on Air China (18 Dec), then directly to San Francisco, USA on Air China (18 Dec). The scheduled layover time is 2 hrs 45 min in GZ and 4 hrs 30 min in Beijing. Is there enough time, especially in GZ, to do the health declaration, frontier inspection, baggage claim, customs inspection, report to transfer desk and security check before proceeding to the boarding gate for the next flight? I have to change planes at each layover and change airlines in Baiyun Airport. Thank you very much.