Yes, milling the carb base flat is just as simple as you outlined. It does not require any in-car measurements.
BUT......
AFAIK, the Blue Thunder “Pantera” intake is the ONLY intake that offers a parallel-to-the-ground design when installed in our Panteras. All the other intakes are designed for passenger cars which install the engine at an angle as opposed to the flat installation in our cars. so those intakes have an angle which positions the carburetor flat when installed in a standard passenger car.
The carb base does not NEED to be milled flat for your Pantera application.
Just about every Pantera on the road does not have a flat carburetor base. The workaround is to fine tune your fuel bowl levels to avoid fuel surging out the vent tubes during hard braking. There is also a modification that can be made to the two fuel bowl vent tubes - connecting the two of them with a fuel-safe piece of tubing that has an air vent hole at the highest point.
milling your intake flat is the ideal approach, but it is likely not worth any more grief in your already frustrating endeavor
Larry