Wouldn't it be easier to put some stand-offs onto the existing mounting bolts to move the speaker away from the wood mounting?
If you cut a new pair of plywood oval rings, screw the new speakers to them, and then mount this to the existing wood with some spacers you'll not have to change the car, and the only difference will be that the speaker cones are a few mm further back from the grilles. if you were concerned about leakage around the gap, you could add a simple shroud to block it off.
Exactly my plan!
BUT the existing ring is too small to do as you suggest, you can see in the last but one photo how narrow it is. So a new wooden plate needs to be glued to the back of the bulkhead and then I can screw a new speaker mounting oval ring to the new plate.
Once the speaker chassis is properly mounted to the ring and sealed air will not be able to leak directly round the speaker. The rear void is big enough ro act as a ported enclosure. It isn't exactly HiFi....
the rear pannel has to come off.
