I had a DZ years ago and put 1 7/8" headers on it because they were a bargain. They were MUCH better than stock manifolds. But I always felt 1 5/8" would have been a better choice. Don't get me wrong. The 1 7/8" pulled hard with that engine, but they were 36" long. And, FWIW, the collector is the most important part.
There is a lot going on in a set of headers. You have a slug of exhaust going out, a pressure wave caused by that, that travels at a given speed because of the temperature of the exhaust gas; then another wave returning up the pipe at a different speed, because the air in the pipe is no longer as hot as the exhaust gas. Each wave has to get to the right place at the right time to make a set of headers "work". Those waves are augmented by primary tube length and collector length. The speed of the gas in the pipe is important; not just getting it out of the pipe. Gas speed, wave speed, and temperature of the gasses all work for you or against you. There are more to headers than "Letting it breath".
When an engine "comes on the pipe", it means the headers are working best at that particular RPM, cleaning exhaust gas out of the combustion chamber without over-exhausting. Exhaust gas dilution is a huge power killer. That's why a good intake valve seal is so important. You must keep burnt gasses from diluting the next intake charge. Likewise, you want to be sure the headers will do their job in pulling as much residual gasses out of the combustion chamber as possible. In my opinion, the best way to do that is by having the gas speed in the pipe high enough to do some work, as opposed to just being blown out into the world.
I'm certainly no expert on this, and from what I have read, neither is anyone else. You can only take one set of parameters at a time, and try to make a set of pipes that work with those parameters. All the figuring will get you close, but only testing will get the best results. My experience has been to err on the small side, because as I said, gas speed must be high enough to make the pipe "work".
There are some good books with lengthy formulas for finding the right diameter and length of exhaust pipes, but it is still cut and try. Seems no one has figured out how to think like an engine.
Joe Sherman says he has made over 740 HP with 1 3/4" headers, and going larger hasn't made much difference at that power range. I will never see 740 in my life at the rate I'm going, so I have to take his word for it. I have witnessed over 700 hp in dyno pulls with that size headers, and the engine builder didn't think he needed anything larger. (NASCAR type 350 stuff from about 15 years ago.)
I'm not trying to say anyone is right or wrong about header sizing. I'm trying to say: Don't pick 2" pipes because they are big and look cool. Do some research and find some combos close to yours that are running well. Go to a track and look around. See what people are running in the power range you are expecting to attain.