The chilean border posts are nice and bored, might let you use their kitchen, living room and wifi if you ask nicely and chat with them. There is a nice place to camp by the river but bring enough food.
'''''Paso Internacional Los Libertadores''''' between [[Mendoza]] and [[Santiago de Chile]] is probably the best option with lots of truck and civilian traffic. You will also pass Mt. Aconcagua, the tallest mountain in the Southern and Western hemispheres. There's a tunnel between the two countries. Goes between Mendoza province and Region V de Valparaiso. Careful, every once in a while it closes due to weather conditions for some 2 or 3 days - check twitter @UPFronterizos before going!
'''''Paso Vergara''''' on the Ruta 226. Crosses from Mendoza province to Region VII del Maule. Argentinian customs is 8km from the actual border. Let us know if you hitched this.