your mechanic is looking in the wrong place. you have two GM L29 motors that have been sold to Mercuiser thru GM industrial engines, with the engines produced in the Tonawanda engine plant, delivered to mercruiser on a crate with heads, valve covers, oil pan, flywheel, starter, and water pump already installed.
The longblock is then marinized by mercruiser by them bolting on their exhaust manfolds, intakes, alternator, distributor, etc. the blocks and heads are EXACT SAME as ANY and I mean ANY GM 3/4 ton or larger truck from 1996-2000 with the 454 (7.4 liter). It is a GM L29 Gen VI big block. between $400 and $700 for a running core, blocks alone for $250 . search car-part.com for a salvage yard near you
The 7.4/454 from 1965 to 1990 is known as a Mark IV motor - 4.250" bore by 4.0" stroke, 2 piece rear seal and about 3 dozen heads over the years (781's and 049's being popular for boats
The 7.4/454 from 1991 to 1995 is known as Gen V motor 4.250" bore by 4.0" stroke, 1 piece rear seal and peanut port heads (really run out of breath at about 4500 RPM)
The 7.4/454 from 1996-2000 is known as a Gen VI motor or L29 in a short deck and L19 in a tall deck. 4.250" bore by 4.0" stroke, 1 piece rear seal, large oval vortec heads (fantastic swirl inducing head good for up to 5500 RPM)
The 8.1/496 from 2001-2009 is known as a Gen VII motor or sometimes called a LS big block. 4.25" bore x 4.375" stroke, 1 piece rear seal, cathedral port heads.
You can also build a 8.1/496 by taking a 454, boring 0.060 over and switching to a 4.25" crank
The 8.2 liter in a Mk IV, Gen V, or Gen VI is a siamesed bore block, 4.5" bore x 4" stroke (this is a GM performance parts block only, never in a production vehicle), however made available to marinizers thru GM industrial motors
Anything bigger than 8.2 liters is made by using a tall-deck block
The ECM will work only with the intake, injectors, etc. that are bolted to the motor for the 1999 and 2000 year, however the block and heads could be from any Mk IV, Gen V or Gen VI motor because the intake, injectors, etc does not know what its bolted to.
I am currently in the middle of a 10:1 489 or 496 stroker L29 build myself with the goal of 480hp on 89 octane with a motor only price under $3k (exhaust is separate). The heads will be my limit at 5500 RPM because they stop flowing enough air then.
for the price your mercruiser mechanic wants to charge you, you could have a set of freshly rebuilt motors with all forged internals, a remapped ECU and about 420-450hp each with a set of stainless marine exhaust manifolds and risers and have $20k left for fuel
ound: Have not heard one that good in a long time 
