Trax, Robart also makes air up/spring down systems and a lot are RoboStrut Ready. I have the Robart 701RS system for my Top Flite P47 kit and 7/16th robostruts. The 701's are air up/spring down 85degree retracts (no idea what the Mustang calls for degree wise). The Robart 511RS and 510RS are also air up/spring down retracts (511 is 85 degree, 510 90 degree) and accept 3/8ths inch struts out of the box.
The Robart 701RS' are 127.99 and currently out of stock at Tower Hobbies. The 511RS' and 510RS' are 126.99 and are in stock at TowerHobbies.com. If you look at the 500 series do yourself a favor and buy the 22 dollar Robart Variable Rate valve so you can control the retract speed, without it they'll just slam up and down possibly weakening the wing structure over time or liberating your rail mounts after a while. Or if your cheap you can just use some wheel collars on the air lines using the set screw to limit the amount of air getting to the unit. You can buy the 701's at Robart.com if you like those better but they are about $40 higher there. The 500 series would work just fine for your Mustang. Robostruts are sold separately and come in all kinds of configurations.