For a big block, when should one consider using 2" primaries instead of 1 7/8"? Is there a certain HP or RPM range?

I wouldn't use 2 inch headers unless the engine was making over 600 hp. Might not use the larger header until close to 700 hp.

Every B and RB motor that I have put bigger primary headers on during engine dyno testing made more HP and torque
I haven't done enough header testing at the track to be able to say the same thing so keep that in mind, dyno testing is just that, testing on a engine dyno, not at the track
The time slip and the spark plugs don't lie

I have 2 inch on my engine and its making over 800 lol. Granted they are short, unequal length fender exit headers but they work.

Sometimes too big of a header is just as bad as too small.

Over the years every time I have had a header on the large side the 60 ft seems to have suffered.

Jere Stahl (Stahl headers) once told me in order to use a 2" primary that engine needs to make a minimum of 550 hp. Since he was the inventor/developer of the equal-lenght/4-into-one header, I think he kinda knew something there. I did some testing at Fremont in the early 90s with my stocker and that pretty much proved right, at least on a stock eliminator car...too big=loss of performance/slower et and mph. I never forgot his conversations nor some of the things he told me. Jere was one inovative and very smart cookie, right there with Harvey Crane, Ed Iskedarian, Bruce Crower, Racer Brown, Gene Winfield and many more of that generation.

lets throw muffflers into this for back pressure.i use pro parts 2 inch headers on my street car with mufflers.stock 440 with 509 cam 906 heads with tunnel ram 2 660s

FWIW, I had:
- 1-3/4 on car when I bought it and kept them when it was a 12-sec car... 450-ish HP
- switched to 1-7/8 when engine got upgraded to run into the low 11s... 550-ish HP
- switched to 2" when another upgrade got solidly into the mid 10s... 600 HP
- keeping the 2" w/ the new engine combo that's making 650+ HP

If I remember right the Moparts Engine masters entry made it's best power/torque curve with some no name 1-7/8" headers. Years ago I spoke at length with Ed Henaman of Headers by Ed. One of the ideas he stressed was not to go to big with the tube size and was a great advocate of equal length tubes.

Ed paid a lot of attention to the transition from tube to collector too.
A lot !
But he made some nice headers.
I bought a lot of parts from him over the years , but never had a combination that he made headers for.

I gave him the details of my motor and chassis and he gave me the recs for tube size and lengths. I then bought the header kit from him. I went down to his shop to pick it up and that's a story in itself.

The other thing on the 2 inch vs 1 7/8 deal is I don’t think you can go to a 2 inch without automatically getting a 3.5 inch collector