Phil,
As you found out the installation framework for the DIN on your RS stereo is not the same as the Clarion. The DIN opening size is the only thing that is universal. After you remove the Clarion from the opening, using a set of "keys" that came with the Clarion, you'll see that there is an aluminum frame that was inserted into the cutout and secured with some tabs that fold over and hold the frame in place.
If you don't have the "keys" for removing the Clarion look on Youtube and you'll see how to make your own keys from a coat hanger. Just type in "How do I Remove a DIN Stereo". Once you have the DIN frame out you'll put in your new DIN frame. Your wiring will not be plug and play because all of the manufacturers use their own plugs. However the color of the wiring is generally universal. Red is positive, black is ground, yellow is constant power, orange is power that turns on an amplifier (I think those are the colors) , brown is a speaker wire, etc.
You can double check the colors by removing your speakers and seeing what colors are on the speaker terminals. Write down the colors to each speaker keeping in mind there is a positive and a negative on the speaker terminals. This is important because if you cross these wires the speakers will play but the sound will be muddy.
If you are using an amplifier the only good way to connect it is with RCA cables and the power wire from the stereo head unit that will turn on the power to the amp so you won't need a separate switch to turn it on. This is usually the wire that runs a powered antennae. There is also another power wire that goes to the head unit that runs the internal memory for the clock, preset stations, and the sound settings etc.
If you want to buy a plug for the Retrosound check www.crutchfield.com they are the largest online retailer for electronics and they may have one. That way you can have the RS plug that attaches to the stereo and a separate plug that you wire all your connections to and these two plugs mate up so you can remove the stereo later without cutting wires.
An amplifier is really the only way to run your speakers. Speakers aren't ruined by too much power they get ruined through distortion which is usually caused by not enough power. The 4X50 power rating that is advertised on a lot of head units is dubious at best and should be ignored.
Quality sound can be achieved in these Speedsters even with the top down. It just takes more power and better equipment. For those that disagree, you haven't ridden in my Speedster. You feel all of the bass notes and you can tell which instruments each of the high notes come from. I drive with and without my music depending on what I'm doing. Often you will see my wife and I singing along at the top of our voices when our favorite songs are playing.