Retro Mechanix features a report on the evaluation of two P-51D Mustangs that were retrofitted with liquid rocket motors. The fuel and oxidizer were held in fixed wing tanks (on opposite sides). The fuel tank held a mixture of analine and furfuryl alcohol and the oxidizer was red fuming nitric acid. Both were pressurized by compressed nitrogen. In the attached photo, you can see one wing tank and the nozzle, which protrudes below the trailing edge of the side Air Force marking. One plane's motor was rated at 1300 lbf for a minute and the other 690 lbf for approximately two minutes.
Both motor configurations were ground tested but never flew. WWII ended and the conclusion was that the propellants were too dangerous and the rocket motors would adversely affect the flight performance of the P-51.