Carb for 340

The calculators are not usually about performance. You're going to have to tune anything at all really, so it comes down to what suits it best. Sure, a 600 is fine on a 318 that doesn't see over 4500 RPM or have much power but a 340 can see gains with a 750 or 850 stock. Holleys are easily tunable and serviceable. Do agree on the run what you have to dyno-tune, if you can get your hands on a 750 to borrow like a shop carb and get to play with the tune that could lead you in the direction of what you'll want to buy.

I've heard possibly 3 SBMs not sound mean. Sounding good doesn't mean it has much cam to it- there's a definitive difference in note but many can't even tell a very small cam's note from a mild one. Poor tuning can make a rough idle too.