Congrats on purchasing a great boat and welcome to Team Talk.
For starters, your engine is a 351 Windsor.
To remedy what sounds like a rich situation, I would hit the basics first:
Check spark plugs...might be pretty black by now.
Do you still have points.......if so confirm correct dwell - don't rely on point gap
Confirm choke opening fully by 90 seconds
Turn mixture screw in until lightly seated and count number. Should be close to 1 1/2 turns
Confirm base timing at 10 degrees or whatever spec says
Clean flame arrestor.
With all that done, I suspect that you have too rich a power valve in your carb. Should be 2.5 though some carb numbers spec a power valve closer to 5.0
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