1- Export the 1080i 60 master using quicktime export, current settings. Now in FCP, create a new sequence ntsce 4:3 60i, and drop the Master HD quicktime movie in the timeline. It should have the Black bars. If you want to go full screen, just double click on the clip and select motion, zoom it untill it fits the screen. You won't loose any resolution since it won't go beyond 100%.
Though I haven't tried it, it might work by exporting a reference file.
You can use that method any way you want. Which is easer for you, better for your project's workflow?
Personally I'd rather edit a master project file in HD. Down convert to SD later. If I ever need to go back and re-edit for some reason, I'd rather have HD version as the master edit so I won't have to re-edit said project twice, one HD, one SD. It's easier to downconvert a copy.