If you read the title and thought Oh no a noob,
well here recently I have noticed that the P-51D cruises at about 350-375mph, when the actual aircraft cruised around 410-430 mph? Can this be fixed?

P-51's did not cruise 410 to 430 MPH! With ords and drops both, the real ponie cruised at 285MPH, IAS, or there abouts, with drops only, about 310IAS, with out drops and normal cruise about 330 to 345 or so. Aces High has it programed about right. Again it all depends on what the pilot wants out of his aircraft, distance or speed! Also your altitude will affect what airspeed you want to make. The figures I quoted are at sea level, 59 degrees F., At 25,000 feet, your IAS is going to go down, but your true airspeed will go up, with the less dense air of 25,000 feet.
A lot of people in this game fly around at full throttle all the time and in the real aircraft, wouldn't be long before the engine started having problems.