Well, I just can't help myself. I keep spending money on the Slant Six even though my current project is "supposed" to be my 408-powered (stroked 360) Barracuda. I scored an Offenhauser 4-bbl manifold on EBay Motors awhile back and decided to get a small 4 barrel carburetor for it while the getting is good.
There are really only two choices for a carburetor on a street driven Slant Six: The Holley 390 vacuum secondary and the Edelbrock 500 AFB (anything else is much bigger than the airflow requirements of even a fairly stout Slant Six). I really like the Edelbrock carbs, but I feel 500 cfm is a little much for a mild Slant Six. The old Direct Connection racing manual recommends the Holley 390 / Offenhauser combination as the "best" street 4-bbl setup, but the information is a little (lot) dated. I decided to turn to the premier source of Slant Six information on the Internet. Searching the site brought up lots of strong opinions on which of the two carburetors is "better," and most of the them were based on the poster's experience (i.e. Holley guys liked the Holley carb and Edelbrock guys liked the Edelbrock). One person did an actual drag strip test, though, and found the Holley to be a tenth quicker in the quarter mile. Based on that, I decided to go with the Holley. If you decide to go with a Holley 390 on your Slant Six, you need to know they make 2 models. Part #0-8007 is a 4160 vacuum secondary unit that sells for around $350, while part #0-80507-1 is a 4150 double pumper that sells for almost $800. Thankfully, the less expensive model is what we want for a street driven Slant Six. If you absolutely can't stand 4160 style carbs, you can get a kit to convert the 0-8007 to a 4150 style carb. The combined cost of the 0-8007 and conversion kit will still be a lot less than the cost of the 0-80507-1.