what is exactly the difference between a "short-block" engine and a "long-block" engine?

also, the new vette engine is always referred to as a smallblock engine in mags...if the engine is 6.0L, what constitutes a big block?

 

short block: the bootom block with internals , but no heads.
long block: the short block + heads (in the VQ case CAMs also).

WRT Vettes, Chevy has always differentiated between big blocks and small blocks, alkso referred to as Rat vs. Mouse motors . The LT series is a modern day incarnation of the small block lineage (started back in late 50s).

Displacement does correlate but does not define alone whether it is a Rat or Mouse motor. You can stroke and punch a small block to well over 400 cubic inches, but it is still a "small block". Were as Chevy once offered a 396 cubic in "big block".

 

In the old small block and big block days chevrolet had only two v8's that were completetly different from each other.
The small block had the same basic block, heads, and external appearance and a HUGE parts interchangebility and was built from 265ci-400ci and could be bored stroked out to as high as 454 with a LOT of $$$$$
The big block was a completely different engine than any small block and most parts on the big block were interchangeable between the various displacements. It was produced from396ci up to 454ci and could be bored stroked out to over 630ci maybe more.

The new chevrolet engines are 100% different than these old engines but still are two distinct engines..
The small block is made in 4.8l, 5.3l, 348ci, 6.0l, and now 427ci for the new z06
The big block is made in 8.1l only for pickups and the boat makers

 

Nissan dealership just put in a long block into the engine to replace the last one which blew due to oil pump going out. My question is do I need to break in the engine as I would a new car? I asked the dealership they said no, but I don't trust them.

 

No need, just drive it the way its going to be driven. Engine break in is a thing of the past, machining and parts are so much better tolerances these days.