Slight modification?
 

How difficult it is to modify the 6x8 speaker opening in the door to make it accept a 6x9 speaker. Also , what other sizes of speaker does that opening accommodate? 6x6? 5.5x7?
Thanks in advance..

No modification needed. Just need the correct speaker adapters. You can fit 8" drivers in the stock location no problem. Again, just need the correct speaker adapter.

This is what I used for my 6X9 for the front speakers.

And the door panel fit right back over it?

Look at my build page link below, you can see how they fit. No issues at all.
https://www.f150forum.com/f30/starti...ercrew-443365/

I provide adapters for free with purchase of speakers along with free speaker wiring harness as well.

Bruce will hook you up!