If you were loading the opposite way I would worry but since you're loading a lighter load (.38 ACP) in the .38 Super brass I see no danger.
That's like loading a .38 Special load in .357 Magnum brass. No danger there but if you load .357 Magnum pressures in .38 Special brass, well then...
That said,
Stars & Stripes Ammo sells 38 ACP ammo so if all else fails you can buy some and then have the brass for reloading. (under production pistol ammo)