water velocity problem

I completely rebuilt a customer's big diverter-T hot water system, all big rads, using pumping away for the first time. Plenty of heat, but job clearly has high velocity problem. All shutoffs full open. Plenty of pipe hangers. Using 18 psi on tank, set at 130F, for 3-stories.
Can I safely add a lock-out type throttle valve after the new red B&G set? Or will the red motor take a motor-rated voltage control without risk, to lower velocity? Rather not repipe anymore at this point.
Thanks, ed.