2010 Rulebook, page 40
4.N ENGINE SWAP:
An engine swap is defined as the use of an engine from an engine design family that was not available as a factory or dealer installed option for a given vehicle year is used.
An engine design family is defined as engines which are made with the same basic material, have the same bore centers, crankshaft supports, deck height, cam location, head mounting, bell-housing and engine mount patterns, etc. All OEM parts shall fit the block without modification.
The use of an OEM or aftermarket replacement engine block from the same design family is not considered a swap. “Crate engines” are not considered engine swaps. Direct replacement aftermarket engine blocks are not considered engine swaps (Dart, World Products, etc.) if they meet the above engine swap design definition criteria.
That should cover it.
DW