If you put mid-bass and tweeters that far apart, you're very likely to run into interesting imaging issues. Like having the three low strings from a guitar coming from the back and the three high ones from the front.
You can work against that by using appropriate processors, but at that point you're rapidly going into four figures for the whole thing and a fair amount of setup time.
I mounted coaxial speakers in custom kick panels in a friend's '58 Impala. They sounded great and imaged very well, and didn't cover the fresh air vents. The project was a lot easier than expected as well. There are tutorials all over the place online, YouTube especially.
If you just want sound, and aren't worried about quality or stereo separation, imaging, etc., replacing the existing dash speaker, and/or adding a center console speaker will be the easiest route.
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