I have those exact speakers in my car- Pioneer 5x7!
they won't fit, your right. So what I did was take some tin snips to the cheap little bracket in the rear and cut off the tabs, and widened the oval hole in the middle a bit. If you're squemish about that, you can pick those brackets up in just about any junkyard.
And yes, the covers fit perfectly. I'd show you them, but I'm waiting to get my digital pics from Bill (with the huge gallery). Just trust me, they fit. fine.
I even went the extra mile and put some foam padding under it all around, to help eliminate the "tinny" sound from the sheetmetal underneath. I don't know if it really works, or if it's all in my head...
-Jesse
'84 blue P-type