Early 72 340, is this one of the de-tuned ones?

So all else being equal ethanol is more resistant to detonation than legacy gasoline. So if you mixed straight 87 octane with 10% ethanol, your resultant anti-knock index will be higher, like 90 or so. But there's no free lunch here. The energy density of ethanol is less than straight gasoline. So even though an E10 blend may be less prone to detonation, the resultant fuel will produce less power if no other change is made, or you'll need more fuel or compression to make the same power. Tune the engine for the fuel that you intend to use. More annoying is the fact that ethanol is more "volatile" or has a lower vapor pressure then straight gasoline. This will make hot restarts on a carburated engine a bit more hassle as the fuel boils at a lower temperature, and it will evaporate out of the tank slightly quicker. On a typical Mopar engine, the actual compression ratio was somewhat less than advertised as the deck heights were generally taller than nominal. Add a modern composition head gasket and a 340 with earlier pistons is probably closer to 9.5:1. Right where you want it. I've had more engines where you could advance the timing to the point of power loss without detonation on 87 pump gas (E10) than I've had engines that really needed premium. But then again I'm at a higher altitude.