OK what exactly did this guy do? Rebuilt a 1986 4.3 V6, or intall a later engine? With the GM 4.3s there are many variations that are very complex over the many years of production. Start with the engine casting # which is on a pad behind the cyl head on the starboard side as its installed in the boat, then there is a serial # on the front of the block kind of right where the head bolts to the block. It will have a letter in it, T means it was built in Tonawanda NY and R means it was built in Romulus Michigan. These 2 bits of info will help you figure out what you have. Then there are other options. Volvo penta also used the long dipstick that goes to the bottom of the pan with the banjo bolt. I'm not sure of the thread differences though and what year they went to all metric. I would not be surprised to find some SAE and some metric on the same engine.
This is why, that when I need a new engine, I'm either going to buy a reman old style 4.3 like I have, so all my stuff fits, or a new GM marine vortec, I will make the modifications that are needed but only do it once. At least with those 2 options you know what you are starting with!