Hi JamesPohl,
There was never a P-51 with a mass of 5,400 pounds, even the prototype. They usually went from 7,500 to 10,500 pounds.
Most performancer figures are calcaulted at 8,000 pounds, where is is an 8-g aircraft. At any higher weight, the g- limit is 64,000 divided by 8,000.
So, at 10,000 pounds it is a 6.4-g aircraft.