Most definitely, a search tuner was available in 66.
If your original radio wasn't a search tuner (didn't have a foot switch or search bar), and your new radio fits, it's a non search tune unit of the same shaft spacing family. Good!!
I am pleased to hear it's sounding good.
FYI - in case you didn't know, with the reverb and/or plain rear seat speaker option in 66, the front speaker was 6 ohms, and the rear was 10 ohms. It seems you already know about the concerns of too little ohmage ruining amplifiers, so you need to make sure hat you get the right speakers.
2 x 10 ohms would be OK, but 2 x 4 ohms would burn up your radio's amp. 6 + 10 would be to spec.
Single dash speakers were 10 ohms I believe.
New correct ohmage speakers can be bought from Electro-Tech, found here:
Classic Car Speakers
I bought one for my 66 and I could not be happier with how it performs with my rebuilt search tuner.