I would just use a manual wrench, it's not like this takes hundreds of foot lbs torque input, as the screw on the clamp gives the mechanical advantage, though some penetrant soaking in for an hour ahead of time, couldn't hurt.And from personal experience that will make air tool purists "rabble rabble rabble," a cordless electric impact affords better trigger control, esp when backing out the screw because you don't get that post-trigger "RRRRR!" free spin
Manual wrench allows to watch more carefully that you keep the pieces aligned and that you aren't bending the clamp. This is in context of being the first time this person/vehicle are meeting up to do this job. Someone doing same job on same vehicles day after day, impact wrench then speeds them up.