My thoughts…. What exact cam is it? We are missing duration @050 numbers, compression ratio, headers or exhaust manifolds.
Given what you have said, the 750 will work fine. The 550 is a bit small and will tip the secondary side more often. While I’d agree a 650 is probably more spot on, the 750 is what you have that will work best. I wouldn’t bother purchasing a new 650 carb myself.