The Chevy engine went through many changes over the years, the first change was in 1980:
Engines 1979 and earlier had a port side dipstick on I/O engines.
From 1980 to 1985, the engine had a starboard side dipstick on I/O engine. Most engines had a two piece rear main seal (Figure A), perimeter bolt valve covers (Figure B), and a 12.75 inch diameter flywheel until the end of 1985.
The 1986 engine was all by itself for the following reasons. The engine still had a starboard side dipstick and perimeter bolt valve covers (Figure B), but it had a one piece rear main seal (Figure C). This also changed the flywheel diameter to 14 inches (exceptions for Volvo Penta) and the bolting pattern of any accessories that bolt to the same bolts that hold the flywheel on, i.e. Engine Couplings etc.
In 1987, another round of changes were made, this time to the top of the engine. Engine now has Center Bolt valve covers (Figure D), and the intake manifold bolt angle has changed on the four center intake bolts, thus making all previous intakes not applicable to the 1987-1997 Engines.
In 1998 came the introduction of the GM Vortec small block marine engine. The Vortec incorporated new, higher flowing cylinder heads and intake manifold along with a roller camshaft and lifters. The intake manifold on the Vortec engine is only held to the cylinder heads with 8 bolts that go straight down. No longer at an angle. Most of these engines used an aluminum intake manifold with a bronze water crossover. This style engine is still current production.