Rivets are much faster in production. They can be replaced with bolts though, I did on mine. I recommend getting a set of junkyard speakers and removing the speaker part from the bracket part, then just do a swap. The factory brackets will hold a standard 6-1/2" form factor speaker perfectly. See the V70 thread linked in my signature for detailed photos on either page 2 or 3.
Concur with Llloyd except it, for me, rivets were easier on the install than bolts. I also lined the speaker opening opening with thinsulate prior to intalling new speakers. There are pics of mine somewhere that show the speaker install using the brackets from the old install.
Found some pics: used the OEM's speaker mounts. A littel ensolite (sp?) around the speaker opening and pop in a few rivets...
Awesome thread - that's exaclty what I needed - bookmarked cause I know I'm going to be doing some more of the things you cover there.
off-topic - the rear spoiler you installed - does it help in keeping the rear window from constantly getting dirty? I have to almost keep the rear wiper on at all times in the winter or I can't see out the back. Never had the problem with other cars.
When you used bolts and nuts - was it a struggle to get a wrench in behind to hold the nut when tightening? or did you mount the bracket first, leaving the hole in the middle for that purpose, and then mount the speaker to the bracket?
They look like the early premium speakers, so I think you're okay. You may be able to upgrade the front doors to the two-way lowers, but you'd need to swap your door panels.
I installed my speakers using the nut/bolt method as well. I drilled out the rivets and got slightly oversized bolts, so that I could thread them into the new hole. I then used double sided tape to hold them in place from the back (wrenched them down) inserted the speaker, and then just throw a nut on and tightened down from the front (using lock washers and regular washers all around).