Tapachula/Huixtla to Zihuatanejo - PLEASE HELP ME

I'm trying to get from the south-west border of Guatemala (the main cities around there seem to be Huixtala or Tapachula de cordova y Ordonez) to Acapulco/Zihuatanejo by bus. I can't seem to find any bus websites that connect one of those main cities in Chipas to Acapulco, or any city on the coast in Guerro. I imagine Acapulco to Zihuatanejo will be facil, but I'd really like to try to figure something out from Huixtala/Tapachula. Please help me!
Also, any idea how long a trip like that might take? The map says it's about 700 miles, but I have no idea about bus speeds or road conditions ...
Any ideas are greatly appreciated.
-Katherine

The trip can be made in segments, not on just one bus. Tapachula to Salina Cruz (or Juchitan), from there up Hwy. 200 to Pinotepa Nacional, and from Pinotepa Nacional to Acapulco, or on to Zihuatanejo. Bus schedules for this trip aren't available online, from what I've experienced. Just find out where the busses (mostly second-class on this route) gather/depart from and find a bus heading north along the coast. When you get to the "end of the line" for that particular bus . . . get aboard a different bus moving northward, etc. The trip will take you a couple of days, more if you want to stop along the way. Have a look at the Maps of Mexico website free online map service for routing information.