I popped in a modern head that does bluetooth streaming and handsfree phonecalls. I added a Rennline phone mount for my celphone and I just stream from it if I feel like hearing music. I replaced all 4 speakers, just removing the crossovers on the doors (they were dead anyway) The weak spot is that I have not yet found a subwoofer that can fit under the passenger seat.
It is definitely convenient to have hands free & nav in the old car. I mounted the mic just to the driver's side of the ashtray.
Rather than ditching the rears, you might look at a woofer-focused solution back there, getting your mids & highs (& stereo spread) from the door speakers. jm2c
possibly helpful pics if you are digging into it yourself:
https://www.joepampel.com/post/it-s-only-audio