Lot of good responses here. Like others, I approach it from the driveability angle. A mild build would be any engine that I would put in a daily driver and not hesitate to take on a long trip. As some have pointed out, more than just engine is involved in this. Rear gearing has a big influence.
As a criteria for a mild build, I would add that a mild build will run easily on 89 octane gas without pinging or need for retarding the timing. Requiring a lead fuel additive would be okay since Mopar V8s didn't start getting hardened valve seats until 1973.