408 build and results.

My .02......

Based on what I see the tq curve doing from 5800-6073rpm(loses 50ft/lbs), along with the air flow dropping off on the last two lines of the sheet.......coupled with a fairly fast rate cam profile and 1.6 rockers...... it appears there may be the onset of some lifter bleed down from 5800 and up.
If that’s the case, then the area of the powerband where you’d expect the racy single plane intake to really shine is handicapped by inadequate valvetrain control.

I think the 5000-5800 range could be slightly improved with a single plane, but I wouldn’t be expecting big gains above that point if there is a control issue.

What size are the headers the motor was tested with?