I think you have to remove the bumper for that. It's a bitch of a job, because of seized screws, especially if they have not been touched for a while. Removing the stripes will leave a nasty looking holes in the bumper. But since you are planning on an engine transplant you have to remove the bumper sooner or later.
The gap will be filled, i think a plastic weld or something in that area, and some racing mesh for the opening in the bottom of the bumper, hard to explain when i got it in my head