Chevrolet Repair: 250 inline six intake, boat lake, older cars

Question
What is the difference between a truck and boat intake manifold, for a inline six 250 ci?
What is a riser on the manifold? The exhaust and intake seem to be one unit, and maybe water cooled.

Answer
Hi Marty,
On the boat, lake water is pumped through a jacket in the manifold to cool it down...probably so it isn't so hot in the boat.
The riser probably refers to a heat riser, which on older cars and trucks diverted the exhaust through a jacket in the manifold to heat the intake manifold part up, so the engine would run better cold.

Van