1998x1080 is the classic 1.185:1 ratio
as everybody is pretty comfortable with a bit of letter-boxing, i'd say that's the way to go.
obviously you'll have to scale the image down, but FCP doesn't offer the best scaling.
i'd say run a minute or so thru compressor with the re-size settings set to "best" and see if you like it.
you could convert all your rushes,
OR, do your edit in a 1998 x1080 timeline, and just convert your exported edit.
a lot less encoding time!
only slight downside is no RT in the timeline, but maybe that's not a problem
is your file true 24fps? (not 23.976)
does Blu-Ray do that?
nick