stock 340 carb size????

340 and 360s came with both 800 and 850cfm Thermoquads depending on the year. Remember that the thermoquad uses the small primary bores for good street manners and then has the spring loaded secondary air door which regulates flow up to whatever is needed, to the max cfm of the carb. That is why they were able to be used on Binder pickups (International Harvesters) that probably had a max cfm demand of 400-500cfm.

That spring loaded secondary makes them very versatile.