318-3, J heads, Hardened seats, and a few other ????'s

Hello everyone. I have a few small block questions for you.

First off, I am looking at a car that is supposed to have a 318-3 in it, is there anyway to verify just by looking at the engine?

Second; Do 73 J heads have hardened seats and how do I tell if they do or don't?

Third; Can I put 340 J heads on a stock 318 without any mods? I know they will bold on, but will it change the compression?

Fourth; How high risk is it to drive the car with modern fuel and non hardened seats?

Thanks, Jeremy