Some of the best locations to optimize imaging for frontal speakers are in the kickpanels, which can be contructed with fiberglass and covered with original carpet, leather or whatever .
The key is to minimize the "path-length difference"- the difference from the distance from you left speaker to your head and the distance from the right speakers to your head. Usually the farther away and lower the speakers are are, the less the difference is and the more stable the stereo imaging is- the perception of a 3-d soundstage with distinct locations for vocals and instruments. Intuitively, this doesn't make sense. Most people in early car stereo competitions started by placing the tweeter high up on the doors and the midrange/woofers separated lower in the door. But physical speaker location has nothing to do with the psycho-acoustical perception your ears and brain uses to create a full "soundstage" from just two speakers. And, no, center channels made popular with home theater usually interfere with proper imaging from two speakers. Any good car stereo install shop can contruct fiberglass kick panel pods. The key is aiming them for the best sound. The advantages are a stealth look, no alteration of the door panels, no cutting any holes, and the ability to remove the new kick panels if you ever want to go back to a stock look.
The disadvantages are possible intrusion into legspace, the time and expense of constructing the fiberglass panels (lots of bondo, fiberglas and even more sanding) and the loss of the vehicle's vent in the original kick panel (which my '63 had).
Another option is "waveguides"-- long and thin horn type speakers that fit under the dash. They give great dipersion and imaging all over the car. checkout a google search on "Speakerworks Buick Grand National" - a famous car audio competition car from the '80s that pioneered this idea.The disadvantages are that they are harder to integrate into the vehicle's cosmetics; they need additional EQ because they aren't they don'y have the most neutral, natural sound, and they take up knee/leg space in front.
The easiest option is a lower door panel location. remove the entire stock door panel, find a location away from window linkage, cut just enough metal to fit the speaker, making sure that any sharp metal is filed down so it won't short out the speaker. If there isn't a solid enough mounting surface, use a plywood or special marine-type plywood (not particle board-it falls apart when it gets wet) for a baffle board. The edges may have to be tapered or sanded, and the borad should be silicone sealed and bolted securely to the door metal surface. Also the entire door should be covered with Dynamat, or other sounddeadening material, to reduce vibrations, seal leaks and solidfy speaker performance (the transfer of physical vibrational energy from the speaker to the door frame wastes sound output from speakers). Once you have the door panel off, you can easily identify different options for hiding the speaker. One is too use special carpeting that can match stock carpeting and still let the sound through. Another idea is to use UV-treated speaker grille cloth glued under an opening in the vinyl or leather, which is peeled back in "finger'-like projections so that the opening edges have a neat, OEM, professional-look. This might also be a time to consider reconstructing your entire door panels to fit and hide your speakers. Another idea is to perforate tiny holes with a die punch in the leather or vinyl over the speaker area underneath. Or if the speaker grille looks cool, use that (although they will rarely fit in with the aesthetics of an old classic car). In my case, after reinforcing and deadening the door metal, I mounted a "plate" speaker that had a rectangular grill with the same dimensions as the lower carpet trim. And the speaker/grille was so simple and beautiful it was once featured on the NY Museum of Modern Art..
Although an array of front speakers in the kick panels and lower door panel would be loud enough (with the right speakers and amplification), a convertible may need some more speakers for more clarity (not always the best idea--the more speakers, the more likely phase cancellations and incoherent sound are). One area in back I have used is the sides of the rear arm rests, which, again were reconstructed and perforated with a circular pattern of die-punched holes-- or they can be covered with grille cloth/ leather/ whatever.
All this takes careful planning. Consult with a professional upholstery expert. Collect pictures from magazines on what you like. There was once a speaker enclosure company named "Z-Box" which made custom-made upholstered speaker enclosures for Porsche and Mercedes that looked like they were stock pieces. They gave me my ideas of what I wanted my rear package shelf to look like Since I'm not the best upholsterer, I had some one else do that portion of the install. I showed a shop pictures of what I wanted it to look like. They made fiberglass mold from my original, covered it with tucked-under vinyl and flush-mounted grille cloth.
Hope this helps a little.