Headers tube diameter.

I ran 1-7/8" on a small block Ford years back. I had studied up on all the sizing formulas and reasons why to use certain sizes and decided on this larger size to avoid restricting the other good breathing parts in the engine: heads and intake. Welding on a set of long collectors, about 21" total, did make a noticeable improvement on the low end torque. Engine was only 325-350 HP tops, with higher torque numbers, and was used over a wide RPM range. So I can't imagine not using 1-7/8' for 500 OR 600 HP. And Yaller Rose's point on size when getting towards 600 HP seems just common sense.

BTW, size fitment was not any problem in that particular Ford chassis, but there were no torsion bar to deal with.