It was actually a fun puzzle to figure out. The song was exactly 125sec long (7500 frames at 60fps), and exactly 1504 beats long, which means if I play a beat every 5 frames, I'd have 4 beats left over at the end, meaning every 1875 frames I need to advance beat by +1 to reach 1504, keeping everything in sync the whole way through. After that, just sync the song+arrows to the chart, and bam! Sync! :D