340 Motor with 360 heads?

I am in the middle of restoring my 1970 Duster "340" and after removing the motor and taking it apart, it looks like 360 heads were put on, apparently the crankshaft and rods are lighter than original because the balancer has been drilled out. I was told that this was a numbers matching 340 when I bought it but obviously I was lied to. Is it common to put 360 heads on the 340 and what are the pros and cons? Is there a name for doing this? Does this make it a "stroker"?

Also, the car has 4 caliper disc brakes in front, was this standard on the 340?Where can I find replacement drums for the back? I found new cross drilled rotors for the front and pads but am a little worried about finding replacement drums for the back.

any help?