I just brought a pair of Fusion 3ways for the shock towers. They are 200w, 6 1/2 inch. I figure I'll take the speakers home and install it myself. When I remove the speaker covers and the stock speaker, I realize the 6 1/2 are too small. So what is the proper size for the back?

1990 GXL.
The fusion sounds pretty good, got them hooked up to my home system of the time being