Using the readme from the soundtrack download, I figured out how to add all of the original N64 soundtrack .mp3s that I had. I went through each level, and customized the soundtrack, mixing the GE:Source music and the N64 original versions - the result is awesome!
If anyone wants a real "classic" experience, I highly recommend trying this out. I used a mix of GE:Source tracks and N64 for all of the new/reimagined maps, but for the "Classic" maps, I exclusively used appropriate N64 tracks for each level, including the old "Multiplayer 1, 2, 3" songs for Complex/Library, etc. - it's a great, nostalgic experience, and it's awesome how the team has allowed this level of customization.