There are gains to be had with the 1.875" primaries on bigger engines & higher hp (500-800hp).. However, a lot of the swaps have limited room to run a larger header. We try and go with the 1.875 most often when we can though.
A few years ago we used an off the shelf Stainless Works header on a 67 Nova we built with an LS7 built to 650hp. Ran mid 11's at 125+ all day long, using 1.75" primaries.
A call into stainless works or other header companies may give you some more info, that's all I got
-Dale