Since it’s a Sondheim show, I’d go with something by Sondheim to show the directors you can pull it off. I’m not to familiar with Assassins, but these songs might work:
“Being Alive” or “Marry Me a Little” - Company (Technically baritone songs, but you’ll be fine)
“Finishing the Hat” - Sunday in the Park with George
“Maria” - West Side Story (ok I know Sondheim only did the lyrics, but I love this song)
There’s probably (likely) better options out there, but those were the only ones I could think of off the top of my head. Sondheim has such an expansive catalog of great songs, so you’ll just have to look around for what you like.