The 3/8 to 5/16 interface with the tighter clamp situation is not a good way to do that--period. No safe long term durability, in my orientation of things.
Take the carb (or the fuel line fitting from it) down to the auto supply and get into their brass fitting section and find a fitting that will mate with the carb and the correct fuel line size. That's the best way to do it and then you won't have to worry about the clamp or the clamped-down fuel line having problems in the future, I suspect.
OR you could get some 3/8" tubing and a regular tubing bender and bend the line from the filter to the carb and the line from the new fuel pump with the 3/8" outlet to the filter.
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