Great to see you here, Eric. On the casting numbers, the factory MR 2X4 intake I tested for my book had the C7ZX 9425 A number on it. I don't know if some of the manifolds came without casting numbers, but it wouldn't surprise me if they did, especially if they came on 65 or 66 2X4 427s. The manifold I tested was from a 67 Shelby Mustang, hence the casting number.
As far as the carbs go, on a 550 HP engine I don't think you would see a big increase in power with a change to the bigger carbs, maybe 10 HP or so. However, you are in the horsepower range where the dual plane design of that intake can start to become a limitation. I think you would get a bigger horsepower boost out of a switch to the tunnel wedge intake, keeping the 600 carbs, than you would with switching to the 750 carbs on the existing intake.
One other point, though, is that if the 750 carbs are mechanical secondary and the 600s are the vacuum secondary carbs, you may get more of a boost in power. Seems like the mechanical secondary carbs make more power with equivalent airflow than the vacuum secondary carbs (usually at the cost of fuel economy).