Since FireRed's song table has half the songs that Emerald does, you'd have about half a megabyte of wasted space from Johto's songs. The most efficient way would be to assemble the S files manually (because I have all the ROM-ready S files) instead of c/p'ing the song data from Emerald to FireRed.
Would you mind making a list of what songs from HG/SS you want in your hack, and what vanilla songs you want to replace?
Would you mind making a list of what songs from HG/SS you want in your hack, and what vanilla songs you want to replace?