Jeremy,
Operationally speaking, there is no comparison between Webers' and PMOs'.
PMO carbs have many useful features that Webers' do not have; ball-bearing mounted shafts, center hung floats, high resolution idle & progression metering circuits, and sight-glass windows to check & confirm float levels.
Webers' are long out of production and the American-made ones (Carter) were terrible.
Used Italian-made ones are good but are NOT cheap to properly refurbish. Aggressive cams wears the throttle shafts & bushings rather quickly.
The 2.7 manifolds may fit but it depends on what size the ports are at their bases. Your SC has smaller intake ports than most 2.7's. PMO does make them specifically for your 3.0 of the correct size.
964 or 20/21 cams will both work.