Keep in mind Ford spent a lot of time with the dynamic DSP tune that adjusts to speed so you must match speaker ohm at least and function (ie no 2 way) don't touch tweeters.
I would put old 6.5 back in or a one way 6.5 aftermarket, and do lots of door sound deadening at a min, disconnect rear speakers, sound deadening in those side panels, and play with EQ first thing after you know your source is high rez 320 kbps min, uncompressed or flac on wideband stream or buy USB. No Bluetooth unless can verify fidelity somehow.
If you are a coupe I'd do sound deadening all over the roof as well it's one massive resonating assembly.
This gives you a solid very low cost base to build from over time. Don't forget to buy trim tools.