Does that dyno service have a good reputation for accurate, and maybe even more importantly, repeatable numbers?

If you’re interested in getting to the bottom of it, “what I would do” is see if you can borrow a fairly decent single 4bbl manifold and a good working 850 or 950 carb and retest it.

Bring that 6bbl with you to the dyno, and if the power curve looks better, and the numbers are more in line with what the motor “should” be making for power...... you can reinstall it and start chasing the tune.

If the shape of the curve, and the numbers are similarly low with the single 4bbl on the motor....... there is something else going on.