best carb for 340?

No matter what brand you get it is all in the tune.

You did not state which thumper cam, gear ratio, stall of the converter and what YOU will be doing with the car. These items have a impact on carb size. While a 750 could be a very streetable carb, it could be a tad big and the smaller 650 would be better. Look at it this way;

The 650 carb will work best when you have a low numerical gear ratio, small cam and you want excellent throttle response for the street.

The larger carb works better with a numerically high gear ratio, stall speed and bigger cams. The 750 favors a higher top end power and speed.

This is not to say a 750 will not work in place of a 650. Throttle response will go down a little along with mileage and best running conditions.
On the flip side, this is also not to say a 650 carb will not perform well where a 750 performs best. The top end power will only be down a few HP.

IMO, iF you choose a Edelbrock carb, I would use the AVS Thunder style carb.
The Holley 4150 is best suited for numerically higher gear ratios, higher stall speed, larger cams and manual transmissions. A vacuum secondary Holley would be better suited for most driving conditions and light racing.

Like above, this is not to say the carbs styles can not be swapped to and made to work well in each other's spot.

If your going to make a driver out of this car, use the smaller 650 carb in a vacuum secondary style. You'll be happier in the long run and down only a few HP on the top end of the rpm scale where you rarely are.