Layered vocals made me think it would be very difficult, but got a few withou many issues. I gave up on Undone and Beverly Hills because the vocals were too complex.
I've got these roughly in order of how well they turned out.
Weezer - Perfect Situation
Weezer - Buddy Holly (I accidentally grabbed the music video version, so there might be a hiccup when there's a break, but I think I covered it well enough
Weezer - Hash Pipe (Easy one oddly... I probably fucked the mix a bit, but I threw some distortion on the vocals to match the original better)
Weezer - Pork And Beans (Very good, but an abrupt octave change on the chorus)
Weezer - Island in the Sun (Octave skipping is janky, but it's not bad considering the input)
Weezer - Say it Ain't So (I couldn't make it go higher without crashing... I think a different voice in the harmony track latched and it goes out of range.