Have just finished doing this exact mod to my 4be. If you use some Corsa 1993 onwards spacer rings, you can fit 5 1/4 inch speakers behind the front door cards without even having to touch the grilles on the card itself.

In the rear, if you carefully remove the grill, you can unscrew the original speakers and fit 5 1/4 inch speakers in the rear as well. You cannot go too proud though or the original tweeter will cause problems upon refitting. You will also have to fabricate a mounting system for the parcel shelf speakers as the originals were held on with bolts through the middle.

Hope this helps.