Many, many years ago (89 or 90), I did something similar in my 78 Regal Sport Coupe. I did a custom dash plate with a couple of different combinations. First was a 4" and two 1" soft dome tweeters in the center with a 4" and 1" soft dome tweeter on each end. Next was 5 1/4" in the center with no tweeter with a 4" and 1" soft dome tweeter on each end. After that I swapped to kick panels.