Native,
What I understood when I bought my 427 SR TW engine (#444) was the 427 SR with the twisted wedge heads is rated at a nominal 525 HP.
That is with the 8 quart
oil pan and a single plane intake manifold and is tested on the Roush dyno with 1 7/8" inch open headers.
Most of the 427 SR TW engines I have seen make a little less than the 525 (mine made 515) and are equipped - as mine is - with a dual plane intake manifold.
Maybe the newer engines make a bit more horsepower?
I recently installed stepped headers and could definitely tell a difference from my original 1 3/4" headers. My new headers are 1 7/8" about 6" then stepped up to 2". The total primary tubes are about 34" long and empty into a 4" collector that is 9 inches long.
I plan to go to a chassis dyno soon to make exhaust changes as well as carburetor changes and will post the results.
Jody