GT40P will be fine with SD. They don't affect manifold vacuum or airflow enough to be outside of the range the map sensor and ECM are looking for.
Twister over on AFM ran the Ford Y303 heads with speed density and the cam I posted above in his car. I actually drove that car between Gateway International Raceway and I-55 Speedway back in 2003. Other than a tendency to hunt for idle speed until it warmed up, it had excellent drivability.
I'm not hating for wanting to switch to a carb, you should see in my build thread where I pondered going carbureted on the 4.6DOHC I'm putting together for my car. In the end I'm going EFI there for the same reason I recommended you staying with EFI, it's actually going to be cheaper.