Guastella was an Argentine international with the Pumas between 1956 and 1960. He was first capped during a match against an Oxford and Cambridge XV in Buenos Aires, on 16 September 1956. His last cap was during the test against France, in Buenos Aires, on 6 August 1960. In his international career, Guastella played 4 matches and scored 1 penalty and 6 points in aggregate.[2]
Between 1985 and 1987, as assistant coach for Héctor Silva, the Argentina achieved its first victory against France, draw against New Zealand and was eliminated from the 1987 Rugby World Cup pool stage. The first edition of the Rugby World Cup was Guastella's last stint with the Pumas.