There was a late 80’s vintage BBC LS6 crate engine on the dyno here the other day.
It was a pretty low use piece that had just been freshened. Cylinders honed, rings & bearings, valve job, new springs, and a comp 282S magnum sft cam.
10.2cr, pump gas, Holley 650DP carb.
Runs manifolds in the car.
Motor was tested with the manifolds and some 1-3/4x3 headers.

Manifolds - 471tq@3600/438hp@5500
Headers - 523tq@3700/463@5600......... better by 52tq/25hp(peak vs peak).

The pulls were started at 3000, where the tq numbers were 448 for the manifolds and 504 for the headers.