340 carb size

It all depends on what the engine want's/can handle. Stock engine, read the charts- 500 cfm. Now play with cam, etc- more carb. But !!! it wants it at the secondaries, not the primaries. This is where most mistakes are made, like Stroker hinted at.

I used to use Holleys, re-jet, check vacuum, new power valve, new pump circuit, etc. Always had to drill holes, with a cam; square or spread. insane.

Play with idle bleeds- touch and go, ruin a carb's orifices. Then a bud bought an Edelbrock, what an eye burner that was. 750 on a 440 with a cam. So learned the Carter carb, ordered the kit, got it to almost not burning eyes at idle, plugs good, better response; ran out of rods and jets to lean the primary one more time.

The whole trick is: timing, then carb. Then smaller primaries the better; let the secondaries give the fuel (they only give the engine what it wants;vacuum).

so it boils down to- smaller primaries, let the secondaries open, when the engine needs it. That's why TQ and Holley 3 barrels where made. But the secondaries still need a tweak, depending on the engine.

If you feel/hear the secondaries cut in; you are getting over a bog, where they cut in too early.