is 11.2:1 compression too much for 91 Octane

Call him (seller) or see him in person and ask some questions about what his procedure was for setting idle mixture. And what the initial timing and timing curve was? Don't comment or say anything negative or any criticism. Just listen to him talk.

Then ask around what year month the motor was built and running in car. And then approx mileage.

If he just set timing to what someone told him. Or had someone set it when the motor was build. There's a good chance this thing has been pinging for while.

If the it's been build for years, it's been pinging for years. With that cam and a loud muffer/axle dumps it's not easy to hear the pinging. So if he heard "rattling", it's been pinging pretty good. And more then he is just hearing.

A friend just took apart a motor with just 10:1 advertised badger pistons. The previous owner was not mechanically inclined showed by the hack work done on the rest of the car by him. Well... the top rod bearing shells are hammered. Some showed lots of copper. Even the top main bearings weren't pretty. This motor had obvious pre-ignition.

I'd be thinking about discounting the motor/car value if this rattling is not a very recent issue.