Stock '66 383 4bbl engine for daily driving ... what fuel octane should I be running? I'm afraid to try anything below 93/premium. Does it make any difference if the gas is 10% ethanol?

I would have said a 383 with stock compression could run OK on 87, but thinking with the ethanol factor may benefit from 92, especially with a more performance minded timing setting.

Try stepping down to something lower and drive it. You will hear a rattling sound coming from the engine if you run too low of octane. With a stock 383, you're not going to hurt anything by trying it.