The diameter of the bar has to do with the energy it can store. A smaller ID bar with a lot of preload will store more energy, allowing the nose to rise faster as part of weight transfer on a drag car. Larger diameter bars will not need as much preload to hold the same weight, so there is less stored energy and therefore more neutral handling in a street driven car.
Do you have to change - no, no you don't. Six cylinder bars will be fine with a big block, but to get the ride height normal you have to preload them more, and more preload means less neutral handling. Look up the weights of the engines in factory stock form.. You'll note there's not much of a difference between them, certainly no more than the weight of a passenger or two. So "have to" is a little misleading IMO. Having built several 6-into-8 big and small blocks, there's not much of a handling difference until you really start driving the car hard.
