1968 Barracuda Dash Speaker Access, Size & Replacement

Before we finalize the interior and place the new windshield on, I'm inquiring about the speaker contained in the dash. The entire dash is not final mounted, as we anticipated needing to replace/repair parts of it. I'm thinking a pair of 4", but looking for people's experiences with this before tearing into it.

That said, I have the following questions:

• What option(s) are there to access the area to remove the old speaker one and install a contemporary one?
• What was the OEM speaker size in underneath the dash?
• What new size(s) have others mounted in there?