In the video he states that it was on a Mustang dyno. I looked it up and they are chassis dynos so this is rwhp.
I have to clarify, the 411 lbs/ft was just one of two similar 408 Magnums he talks about. Both 10.3:1, PIE stage 1 heads, Kegger mods…Difference is cam. The other engine is more impressive with 467 lb/ft.