gmachinedart1,
See the last paragraph for an opinion based short answer to your question. Feel free to skip all the jargon in the middle if you are not interested.
Unfortunately, this is an extremely complex question and there is very little info given. The link from 304-dude and the tip from amcenthusiast will get you close and that may be all that can happen here. I'll breakdown some points below greatly simplifying some of the theory and issues presented above.
Disclaimer: I am not a professional, but i do know enough to make an idiot of myself. Feel free to correct me if I am incorrect, I will gladly accept more knowledge at the expense of my pride. None of the following is because I think you don't know this, but for posterity's sake I figured i'd post enough for you or for someone who finds this post to be able to at least know what terms to search for if you want to dive deeper. This is a very deep rabbit hole full of convoluted engineering theory if you choose to follow it. But now at least people know where the entrance to said rabbit hole is....
First, as amcenthusiast pointed out, 7000 rpm is no cake walk. You really have to know what you are doing. Not saying you don't, but it's difficult to say when we only talk a few sentences at a time on the forum.
Second, a bit of simplified theory. Holding engine size constant (and assuming you are at least in the ball park on primary size), a smaller primary will give better low-end and mid-range power and torque, a larger primary will give better top end numbers. The whole goal is to get mass (exhaust gas) out of the engine. Velocity and flow are the ways to do that. For a given engine speed, a smaller primary will have a higher exhaust velocity (same mass of air thru a smaller area) and that velocity can be thought of as an inertial force. Meaning the exhaust gas will have more momentum behind it to help evacuate the cylinder. So a smaller primary helps when engine speeds (and exhaust volume) are lower since they keep velocities higher. As engine speeds get higher, this smaller primary may not have the area needed to support the needed flow to evacuate the exhaust gasses effectively and will pose a restriction (like trying to breathe thru a straw) and hurt top end power. Conversely, a larger primary will pose less restriction at higher rpm, giving better flow and better top end power, but will be a bit lackluster in the low and mid rpm due to the exhaust gas velocity being lower since the primary is larger. This will kill that "inertial momentum" that helps to clear the cylinder of exhaust gas.
Third, any change in the cross sectional area of the exhaust path causes pressure waves (good or bad ones). I'd recommend the article in 304-dude's post as a more in depth primer to much more nasty math. But basically, any time there is a change in the cross sectional area (port size to primary size, stepped header, primary into collector, etc), there is a corresponding pressure wave (positive or negative depending on if the size change is bigger or smaller). This pressure wave can either help or hurt power at specific rpm points and those with really big brains and wallets will 'tune' their exhaust using this theory to help increase power at specific rpm points they deem most useful to the type of racing they do.
Fourth, Ken_Parkman is the only one to offer a data point (so far). And he gained 15 peak hp by switching to a 1 7/8 primary but lost some in the mid-range because of it. His motor was also 50 cubic inches larger than yours (1 7/8" may be too big for a 360). And we have no idea if the exhaust was the limiting factor. Meaning he could have gained more if his intake was completely optimized, or it could have hurt him if it was the reverse. It's the engine combination that determines what mods are most useful. Also, 15 hp is also only a 2.5% difference at the peak. For a road racer, you need to look at average power in the rpm band your motor will be operating at on the road course. 15 extra hp from 6500 to 7000 will not help if you give up 10 hp from 4000-6500. Think of average power (area under the hp curve) over the rpm range you will see on track as a better measure than peak.
Basically it all boils down to how much effort you want to put into this. Yes you can hire someone to model you engine and tell you their best guess on primary size and where to put the steps. Yes you can spend $$ on dyno time testing multiple headers. However, since you are building a double duty car and not a class specific race car, i'd get the most convenient exhaust i could afford. Meaning I'd look for a quality header that was easy (as can be) to install and remove, that cleared all steering and frame and suspension components, and just drive the car. Unless you are a professional race car driver, that few percent power increase (or decrease) will likely not be the deciding factor in your lap times. Your experience, suspension set up, tire characteristics, skill, and time on track will be much larger influencers of your lap time.
Hope this helps. Not trying to put you down. Just trying to get info out there for anyone.
TLDR (too long, didn't read): It's complicated. I'd look for a quality header in the 1 3/4" range and spend my time driving the car and being on the track. If you are not a professional race car driver, time on track will be much more beneficial to lap time than a couple % increase in power.
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