At that level, I'd definitely think a 2 1/8" would be optimal...
Many of your decent street/strip 410ci type small block strokers run 2" headers...the 2 1/8" is perfect for a big stroker presumably turning a little rpm to make those kind of numbers. Light car, stick...I'd definitely go with the big tubes.
You can certainly make 750fwhp through 2" primaries. I've tuned blown small block Mustangs making more than that through 1 3/4 primaries. It can be done.
As far as would bigger be better? Almost always, as far as horsepower is concerned...but I'd be willing to bet the difference between 2" and 2.125" on a 750fwhp engine would be less than 20fwhp holding everything else constant. It certainly isn't badly choked with 2" primaries.
How many times have you seen a car produce 300fwhp through dual 2" exhaust? I've seen it more times than I can count. You're talking about 8 2" pipes. Surely you can make 2.5X that much power through 4X as much flow potential.
Since you didn't specify what cylinder head you have, only hp figures I would have to say to measure the longest side of the exhaust port opening ( flange side ) and stay within a 1/16 to 1/8 max above that number.
example: if the longest side of the exhaust opening is 2" then you would want to run a 2-1/8 primary pipe. That way you will have a 1/16 on both sides of the longest side of the port opening. The primary pipe should be nothing more than an extension of the port opening, This will promote better exhaust gas flow. Same as a porting guy flowing your heads with an extension pipe to make the numbers look better.
Just my humble opinion.
If you're buying headers for the first time with this application I'd say definately go with the 2 1.8" primary headers.
If you already HAVE 2" primary tube headers I'd say the possible minimal power gain is not woth the expense for the larger header and just have a 4" collector installed on to the present 2" headers.