[Car Nut]

The plate you are talking about, was meant to replace the metering block and jets in the secondary system on a 4160 carb, the holes were a predetermined size. You can buy a new secondary metering block to replace the plate as well as longer screws and transfer tube needed if you add new metering block. It's been a while since I have played with one of these plates and I don't think drilling it out and installing a jet is a great idea. New plates with different hole (jet) sizes are available for tuning.

It could be that your miss or stumble at low speed when you floor it is actually "bogging". Or maybe your accelerator pump is not working properly. Ignition timing might be off too. You can an adjustable style vacuum actuator (for the secondaries) and adjust the secondaries so they open slower (or faster).

The Holley carburetor website has a lot of helpfull information.

Also the #61 jets sound a little small.

A 650 cfm carb should be a good size for a mild 351. I wouldn't go to a 750 unless you have the cam and heads to go with it.