1) short block - the block itself, internals may or may not be included
long block = block, head, and internals; manifolds, distributor and accessories may or may not be included

2) a race engine may not be streetable because of a high octane requirement, high emissions levels, poor fuel economy, non-street-friendly powerband, etc.

3) Detonation is when the air/fuel mixture in the combustion chamber is ignited early(and not by the ignition components), usually from excessive heat or lack of fuel(running lean).