This border is only open between December and April, and will close when weather doesn't allow for safe crossing. Check before you go. On normal days, it is open between 7:00 in the morning and 17:00 in the afternoon.
This is the (in)famous Paso Agua Negra border crossing between Argentina and Chile. More than 100 km of it is dirt track (ripios) and there's a total distance of 180 km between border checks. The pass and highest point on the road lies at 4.780 meters above sea level. Depending on your history with mountains, you might get altitude (altura) sickness. There's many glaciers in between and some impressive ice formations. They are going to blast a long tunnel through the mountain, so they can keep the border open year round and lighten the traffic load of the Paso Internacional Los Libertadores south of this one in Mendoza province. As long as Chile doesn't give it's OK for the tunnel, it will remain the beautifully brutal road it is now.
To hitchhike this border, one can either go to the YPF gas station in Las Flores or go directly to the Aduana y Migraciones. Nearly all the cars fuel up before making the crossing, so it's just a matter of finding one that has an empty seat at this gas station. Most drivers will ask you if you carry any drugs with you, as they're pretty scared. Please don't take (what governments may decide are) drugs with you, for crying out loud. Even when they don't have the dogs that sniff your stuff.
Be proactive with asking for rides, as the stream of cars thins out towards the afternoon. Just driving the crossing will take three hours, not counting the myriad hours lost waiting for slow border people to check everything. Mind of a Hitchhiker spent three hours at the Chilean side of the border (named Juntas del Toro) and only got picked up again by her original drivers who went through customs 3 hours later. This was during the height of the summer holidays in early January 2017.
At immigration and customs in Las Flores, your drivers will receive a little form with how many passengers there are in the car. Make sure you're on that form, as there's some military dudes 50 km from the border check whose only job it is to check this paper and match the number of people.
A backup plan for the Chilean side of the border crossing is asking the staff to give you a lift. The Chilean border complex Juntas del Toro has loads of people working there in shifts, so there's both people coming in and leaving who don't have their cars packed full. Even when they stole all your veggies, being nice to them pays off.
The nearest exchange office in Chile is in a place called Vicuña in Coquimbo region. There's also an ATM there. You'll also come by the road to Elqui Valley (Valle Elqui), which also has an ATM and a nice hippie community. This is also where Chile's most famous Pisco comes from, if you're into alcoholic drinks.
South towards Calingasta, Barreal and San Juan city
From the same YPF gas station, it's only a 1 km walk to be outside of town. From here, traffic only has to pass through Iglesia to go to the empty stretch of the Ruta Nacional 149.