I assume SER# is the only true way to tell?

yep! I think besides stickers, the exterior of engine is the same all around...

you cant always rely on serial numbers,they can be bolted on a 454 block,,,casting numbers on the back of the block dont lie..they will tell you its a 8.2 liter..

I believe it is stamped on the side of block also

Drive the boat and tell us how fast it is. Speed don't lie.... GPS of course

Its a DCB MAch 22 with a 500HP carb GIL headers. and runs 78 MPH on GPS.

Stock drive height, and DCB said would be faster with a 1" shorty.

There should be a large 8.2 cast in the side of the block. Also I am pretty sure all HP500's came with roller rockers on studs so If you have those that is a good start.

Look for the 8.2 stamped on the block, in the freeze plug area. Merc also made a HP450, that was a 454, but almost identical to a HP500 spec wise, other than the Cubic inch.

Even if it says 8.2 on the block, it will be a 502, but can have anything inside it as far as crank, rods, pistons, cam, etc. Tearing it down is really the only way to know anything for sure.

If it's never been opened up, it will have tie wire and a dab of silicone on the port side rear Int manifold bolts, and on the crank pully bolts.

Your doing good on speed, should be at least 500HP.