The Ford 3.8 M90 is a bit less efficient than the flow maps posted, it's Gen 3. not sure how much worse.
As to carb size, running anything smaller than the original TB (60mm = 2.36" ID, about 725 CFM @ 1.5 Hg.") will reduce boost, reduce power and increase internal temperature. Roughly, you can estimate your carb size by the usual:
N/A CFM = displacement × peak power RPM × efficiency ÷ 3,456
Now multiple by the pressure ratio:
PR = (atmospheric pressure + boost) ÷ atmospheric pressure
E.g., at sea level and 10 psi:
14.7 + 10 = 27; ÷ 14.7 = 1.68:1
If this is for Bonneville and you have the same boost, your PR goes up, duh.
This gives the carb flow in CFM, but at a pressure drop higher than suitable for max power. This is a Holley-endorsed formula and it gives conservative results for typical V8 use, so I suspect it works near the standard 4 bbl. 1.5 Hg" pressure.
I'm not sure exactly what the Ford intake port looks like inside the case, but the transition from the TB area to the lobe entry is by far the hottest area (indicating a flow problem). I'm not sure if enlarging the port straight through will improve this at all, although the later Ford has a bigger port externally.
The easy part is the discharge - think shape, not size.