The front speaker is held in by a bracket (several pieces). The design varies from year to year. The front of the bracket has a tab with a rubber pad that fits into the inside curve of the dash. The bracket at the rear fastens to screws that are inserted from under the rubber windshield weatherstrip. You should be able to feel two holes about 6" apart under the dash beneath the windshield. I had no luck raising the weatherstrip enough to access these holes. Sheet metal screws are supposed to go through the holes and into spring nuts on the bracket.
The aftermarket dual speaker does not fit the OEM bracket I had, and without the rubber bumper, you have a problem, but perhaps a piece of split hose could be fit in there. I did use the rubber bumper on the front tab of the dual speakers, and made two simple hanger brackets from some heavy perforated steel to support the two back corners of the phenolic speaker base. As along as it holds the speaker firmly to the front where the rubber pad fits, all is well. I had to use two sheet metal screws through the firewall, since I could not access those holes under the seal.
Clear as mud? Get a factory build manual and you will see the numerous brackets used over the years, If you have an OEM bracket, there are speakers that are similar to the original....if you can get to those screw holes.
