Mark is correct regarding the boat builders not necessarily using year correct engines.
And I agree, the Hot Rod forum guys are fairly well versed in this. Just don't use their build techniques for Marine cruiser use.
Apparently the Martec web site has been taken down for some reason. However, somewhere out there you'll find an archived version of it.
Martec did a very good job at listing and identifying SBC cylinder blocks and cylinder heads via casting numbers and/or serial numbers.
In 1986 and 1987, GM made the change from the 2pc to the 1pc rear main seal for the 5.0L and 5.7L engines.
Apparently the 5.0L and 5.7L were one year behind the other, and right now I don't recall which one followed the other.
This is clearly identifiable by the casting number at the rear Port side of the blocks, which includes the engine displacement.
This one is a 5.0L.
Here's where we'll find the earlier 2pc rear main seal engine numbers.
Most all of your components will interchange.
Cylinder head exhaust ports and bolt pattern are unchanged.
Flywheel cover flange and bolt pattern remains the same.
Ignition distributors should interchange for the standard LH rotation engines.
153 tooth flywheel = straight across bolt pattern starter motor.
168 tooth flywheel = staggered bolt pattern starter motor.
Engine circulating pumps will interchange up the engines with composite timing chain covers.
Exceptions will be:
Oil pans are different between the 2pc and 1pc rear main seal engines.
Flywheel crankshaft flange and bolt pattern changes between the 2pc and 1pc rear main seal engines.
Intake manifold bolt angle changed at some point.
In/around 1991 or so, GM discontinued the mechanical fuel pumps, of which means that these blocks are not drilled for the fuel pump push rod.
As the year models become even later, there will be more changes that hinder some parts interchangeability.
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