I don’t actually know how they’re numbered in FE8, but you should probably actually look at the instrument table so you know which instruments exist, and which don’t. Basically, if you didn’t already hear it in the vanilla FE soundtrack, it doesn’t exist.
That said, the best way to figure this out is to actually listen to the soundtrack and pick out which instruments you like. Then you can just grab the pointers for those instruments and customise your instrument map to your liking.
It’s important to note that a lot of the bass instruments are tuned down an octave so you’ll need to push them up an octave in your MIDI. The default electric bass and timpani are such examples.
I’d need to see the FE8 native instrument map to say anything more
Ah yes, and if you’d like to import some custom instruments, Mother 3 and the Advance Wars series generally have instruments that match the timbre of the FE games quite well. The FE11/12 instruments actually work quite well too, but are just a little soft so you’d have to adjust the volume for these tracks up about 30%