Chet,
How far do you intend to go? The type radio head unit and amp power is crucial to determining what speakers you can and should choose. Keep in mind that sound from rear speakers will be buffeted by the wind when driving with the top down and front speakers will be dominant. If you intend to install rear speakers, consider boxed units with built-in amplifiers that can be unplugged from power and radio connections and removed.
Also, you will need to make a choice on the presentation you want to make with your radio (how obvious or inconspicuous and time-consistent with your Healey model you want it to be). Although you can install a modern head unit, equalizer, amplifier, and a host of speakers (midrange, tweeters, and sub-woofer), you would need to determine how far you really want to go and expensive components placed in the wrong environment will not deliver the sound you may be after and paying for.
If you want decent sound from an inconspicuous time-consistent radio, then place 2 small speaker boxes above the heater duct and I expect you will be happy. If your radio supports more speakers, you can even add to the fronts by installing an additional 2 removable box/amplified speakers in back. Or, you can do what I did and install a replicated console (original on shelf) with the needed dimensional modifications to accept Bose head and equalizer, acoustically separated midrange speakers on each side of the console, tweeters at the ends of the dash and an amp supporting my subwoofer inside the spare in the trunk. Since most all was purchased from a salvage yard and came out of an Acura TL or on sale, the cost was reasonable and the sound perfect to listen to the Beach Boys when driving with the top down. But my approach is my approach and most all others have taken a different path and are as happy with their results.
My thoughts,
Ray(64BJ8P1)