Mahler's Eighth is quite an unusual symphony, particularly with the massive choral and vocal parts. Mahler has several symphonies with singing in at least one of the movements. Mahler's Symphony 2 is probably the closest to the Eighth. I would also recommend Verdi's Requiem and Brahms German Requiem to explore (neither are traditional religious requiems). Of course, Beethoven started this very large form symphony with chorus and voices with his great 9th symphony. Related thematically, with soloists and chorus is Berlioz Damnation of Faust, also a very large scale work.