The door speakers are round (6.5"), not oval, so you will have to make custom mounts to install 6x5 speakers in the doors.
The original round speakers are riveted to the plastic mounts.
If you peel the rubber away from the outer rim, you will find the rivet heads below it. Drill them out to separate the speaker from the mounting.
Better getting a good set of 17cm speakers. Plenty of recommendations in past threads
Peter