Do you boat in an area where you will have a marine inspection. Boat carbs have to be vented correctly to be legal and you can not only be forced to fix or replace what ever carb you install with a legal on but also can be given a ticket to the marine troopers ball “ not so cheap ticket”
I’d look for a carter or quadrajet thermoquad spread bore in the 500 to 600 CFM range which will be plenty of carb but small enough it can be adjusted to work on your engine. Fuel mileage when cruising may actually improve with a spread bore as the primary bores can be more efficient than a two barrel with large throttle plates however actual full throttle performance may be less with a 4 barrel which is something that may be hard to grasp but it boils down to air flow fuel atomization and proper CFM for a given engines needs. People often convert to a 4 barrel for the wrong reasons and find out that they didnt achieve what they wanted.
One of the reasons a carter or thermoquad is the fuel bowls are smaller the floats take up most of the room so the amount of stored raw fuel in the carb itself is less meaning if a fire breaks out there will be less fuel to burn at the top of the engine when compared to say a Holley with separate fuel bowls they store a lot of fuel.
If you really just want a 4 barrel then find one to meet marine specs get an air cleaner that will pass fire codes and do it. If you just have carb problems those two barrel carbs are pretty straight forward as long as there not damaged or rotted out. They usually produce more power on fast throttle changes than a 4 barrel in an engine your size and HP
