Seat Guru is outdated.
Before you book it will show the aircraft (which very often changes, but is of course more reliable the closer you are to the trip).
The best you can do is to search for the route on Flight Aware and see the probability of each kind of plane by looking at the flight history. MAD-GIG seems to be always A340-200 (old J), while MAD-EZE is more often A340-500 (new J).
The only planes with the old cabins are the A342 and the single remaining A346.