I think the 600 is typically vac, 650 is spread and/or mechanical. Also ***ume you mean side hung float 4160 style. https://www.holley.com/products/fuel_systems/carburetors/ The brawler has 650 square bore, vacuum secondary, but is 4150 style. there are square vac 4160 style but they are 600. Is the 50 CFM the issue?
yeah, this is the typical old fashioned "cheap" holley 4bbl carb, it looks a bit different than it did 60 years ago. And the price went up while you weren't looking.
A lot of the early ones were listed as 585 cfm which were used on 271hp 289 K code Mustangs as performance carbs. Mines is from a 60’s International big truck…… so much for $$$$$ high performance. Today unless you want this exact Holly you are better off with an Autolite look-a-like that Holley made and now Holley makes them for Summit as 500, 600, and 700 cfm and are reasonably priced.