I believe I'm right in saying the carburetors in the Merlin 66 (Packard V-1650) powering the Mustang don't cut under negative G
You're correct
treaders. US produced engines with a Bendix injection carburettor whereas the UK used the SU carburettor, the -ve "g" cut on UK engines being caused by the float. The cut was a two stage affair, first the fuel was forced to the top of the float chamber which exposed the main jets to air causing a lean cut, secondly if the -ve "g" continued the fuel being forced to the top of the chamber the float would now float on the fuel surface in the reverse sense ie at the bottom of the chamber, with the needle valve now wide open you now suffered a rich cut.
Another indication our friend knows little.