I don't think "article" is the right word for "base". What they were debating in February 1855 was the second of twenty-seven "bases" proposed by the constitutional committee. These were really principles which were intended to serve as a basis for drafting the articles (much more numerous and each much more limited in scope) of the 1856 constitution (which was never actually passed). The word "principle" has been used by historians, and it is what I would use. "Proposal" is a possible alternative.
On the singular "ha sido", which troubles Peter, the following, from the RAE's
Diccionario panhispánico de dudas, may help:
"Cuando el sujeto y el atributo son dos sustantivos que difieren en número, lo normal es establecer la concordancia [del verbo ser] con el elemento plural [...]. No obstante, en algunos casos es posible establecer la concordancia también en singular [...]: «Las migas ruleras es un postre que se reserva para la cena» (Vergara Comer [Esp. 1981])."
http://buscon.rae.es/dpdI/SrvltConsulta?lema=ser
I think the point here is really that Sagasta was speaking extempore and his attention was focused on "barrera", and this made him use a singular verb.
The translation workplace 
peer agreement (net): +2
