North towards Calingasta
This is the RN 149 northward. Walk 1 km northward of the YPF gas station, where the town sort of ends and the fields start. There's a generous shoulder here. It's only 40 km to Calingasta, but make sure you take the east road and not the west road through a town called Tamberías. If your ride is going that way, get dropped off at the crossing in a town named Sorocayense and catch another ride that will take you to the correct side of Calingasta, where there's a Bandera Blanca gas station for continuing on the RN 149 and the bridge into the central part of town 1 km away.
At the south end of Barreal is a police checkpoint on the RN 149. Catch a ride from there to the Province checkpoint at the crossing of the RN 149 with the road to Parque Nacional El Leoncito and the astronomic observatory CASLEO. It's a large set of buildings with a giant roof for shadow and military dudes who are likely to give you access to the bathroom and a water source. If you're lucky, someone will travel all the way to Uspallata directly from Barreal, but from here you have a great filter to only find traffic going to Uspallata. The road is paved till the province border (Limite con Mendoza), where the fun bumpy 40 km begins.
Accommodation and Sleep
Mind of a Hitchhiker asked the police and bomberos (fire people) if she could crash in their (GIANT) garden or in a corner in the building, and was told no. Their building is across from Barreal's only gas station, the YPF at the north end of town.
There's also a Camping Municipal at the south end of town, where a night's sleep costs you around 30 pesos (January 2017). This might be relevant if you continue hitchhiking the 115 km southward towards Uspallata and Mendoza province.