Guys,
this is a good discussion for someone seeking max power and torque numbers, but the application referenced above is for a car that is primarily street driven, which based on the stout engine Robinson is using, not many street tires are going to handle well any way.
Since headers are (nowadays) a big investment, I'd recommend going with the 2" primary to give you a platform that can support future upgrades - like larger flowing heads, induction, carbs, etc, however, if finances are a concern - I'd run whichever one fit my budget. I've ran them both over the years, and the 1 7/8 primary did seem to pull (a little) harder on the bottom, but then the 2" pipes pulled great through the mid and higher rpm range.
From a purely street (driven) perspective, I doubt you could feel the difference, as both will run just fine - especially with 500 cubes on tap. Also, MeepMeep is correct; the typical line of thought is 1/8" larger than your exhaust valve, which means either will work just fine.
Anybody out there remember, just a few years back, that we used to have the same conversation about whether or not to run a 1 7/8 or 2" primary with....our 440's ? With 500 + cu. in. being the norm nowadays, you'd think the discussion would center on 2" primaries or larger ?
Again, just my personal experience. Good discussion guys.
Southernman