I think that I have already commented on this elsewhere, but I am listening to it now, and I thought that it was worthy of a "revise and extend."
Bernstein's Tristan und Isolde is (last check) the longest recording of that work, and it also has singers that can't pack the gear. Listen, for example, to Peter Hofmann's "Nur eine Waffe taugt" from Levine's Bayreuth Parsifal if you want to hear how underpowered he was as Wagnerian Heldentenor (pace Boulez). I am not sure why Bernstein let him be cast, but that's between Lenny and God now.
Lenny's Tristan aside, his Götterdämmerung excerpt with Eileen Farrell ("Starke Scheite...") shows that he had the understanding to be a great Wagnerian. In the mid-1960s, his tempi had not become the stretched, lugubrious mess that they later became. He shows a sensitivity of Wagner's demands, both on the orchestra and the singer, that allows him to modulate the tempo as he needs to make the piece work.
It makes me wish that he'd set a Ring to vinyl at some point.