There is a code stamped on block on a pad on the front passenger side. It will read something like this:

T0216xx – the xx is the key here as in 1966 all 396 engines had one of the following codes

ED – 325 horsepower w/manual transmission
EF - 360 horsepower w/manual transmission
EG - 375 horsepower w/manual transmission
EH - -325 horsepower w/manual transmission
EJ - 360 horsepower w/manual transmission
EK - 325 horsepower w/automatic transmission
EL – 360 horsepower w/automatic transmission
EM - 325 horsepower w/automatic transmission
EN - 360 horsepower w/automatic transmission

Any other code would indicate a "non-stock" engine, which might not be a bad thing, i.e., if it was replaced with another big block that may be just fine unless you're looking for a "number matching" car.

Good Luck,
Glenn