I have a '73 280 Sedan with the 2.8 I6 M110 engine. The Solex 4A1 carburetor was giving me issues, so I purchased a new Holley 390 CFM #510-0-8007 carb to replace it.

My mechanic said it does not bolt directly to the intake manifold and needs an adapter plate, is this something I can purchase or does it need to be custom made? Would any of these work? https://www.holley.com/products/fuel_systems/carburetor_components/adapters_and_spacers/

I found the following links regarding this subject but I'm still lost on what I need to purchase to make it fit.
http://www.peachparts.com/shopforum/vintage-mercedes/190015-holley-350-cfm-2-barrel-replace-solex.html
POVO gets a Holley conversion - Topklasse Discussion Board

You will likely need to make something.

You need the new carb base gasket and the old carb base gasket as a pattern. Any decent machine shop can whip this up using either a stock holley spacer or just a piece of 1/2" thick aluminum.

You could even make one from aluminum stock , a hole saw , drill bits and thread tap.