headers vs manifolds on a 318 2bbl difference ????

In this article before adding cam and 4bbl to a stock low cr 2bbl 318 with headers dyno 189 hp @ 4000 rpm, they never did manifolds but guess it would be 10 or so less hp.

https://www.motortrend.com/how-to/mopp-0312-318-long-block-bolt-ons/