[moore_rb]

Holley power valves are typically stamped based on how much vacuum is required to open them, but I've never seen one stamped with 3 numbers - Usually there is one letter, and 2 numbers.

Based on that, I suspect that the one you have stamped 451 is probably actually an "I-4-5" - If I'm correct about this, then that one would be set to open at 4.5 inches of mercury, and would go with your secondary metering plate.

For the other one- Are you sure the "8" isn't a "G"? I guessing that it might be a "G-6-5", which would open at 6.5 inches of Mercury, and would go on your Primary metering plate. I doubt the 6 is a 9, but even if it is, 9.5 is still a higher number than 4.5, and so it would still go on the Primary metering plate.

When your power valves are 2 different sizes on a vacuum secondary carb, the higher value power valve typically goes on the primaries, so it will open first as vacuum falls under hard acceleration, as a general rule.