However the distributor is mounted on the front of the block instead of the rear.
unpossible. Buick *never* made a Nailhead with the distributor at the "front" of the block. the 215ci and 300ci had valve covers stamped to resemble those on a Nailhead but they are NOT "Nailhead" engines. they are what is considered "Small Block Buick" designs and there are significant architectural differences between a SBB and a Nail.
remember, for our purposes the 'front' of the block is where the fan / timing cover is located, regardless of how the engine is oriented in hull. so, as Bob says, verify that the transmission bell housing has not been oriented towards the bow.
of course, the quickest way to differentiate the designs is that the Starboard side cylinder bank ( like a Pontiac ) is the forward bank on a Nailhead while it's the Port ( or "driver's side" in a car ) side cylinder bank that is forward on most other GM engines. and it doesn't matter how you rotate the engine on a vertical axis, this will always be true.
turning a Nail around 180 degrees will mean that while mounted in a car the #1 cylinder would have been furthest forward on the passenger ( or Starboard ) side, your boat "reverse mount" placement would put the #1 cylinder as the rear most cylinder on the Port side.
IF this is a SBB, then the engine is in the normal timing cover to the front orientation and the #1 cylinder will be the forward most cylinder on the Port side.
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