Engine Terminology

Complete package - includes drive, transom assembly, power steering, trim pump  and complete engine. This is ready to go in the boat, and is the same thing a boat builder buys.

Bobtail - a complete engine with all accessories, ready to run. Does not include drive, transom assembly (or in the case of inboards a transmission)

Long Block - an assembled engine from valve covers to oil pan. Included block, heads and pan. You must transfer over accessories like intake, carb, alternator, etc. Sometimes called partial engine.

Short Block - block, crank, pistons, cam (Mercury does not sell inboard or SternDrive short blocks)

Mercruiser Models

For the most part, Mercury does not use base engine manufacturer or cubic inches to identify their products. Early models were based on horsepower ( 260, 330, 228, etc).  Most of the new production stuff is based on liters ( 5.7, 4.3, 7.4 etc). Then just to confuse things most of the high performance engines are either cubic inches (502 MAG) or horsepower - 800SC. 

There is a big difference between a 7.4 and a 454 MAG.

 In any case, when asking for parts we don't care what GM calls the engine, we need to know what Mercury calls it.