hyd roller cam retro ?'s

Yes, Rhoads does do variable duration lifters.I don not know if you know this.Rollers,with quicker ramps,have much better vacuum than flat tappets of the same size.I do not know if they do a retro fit lifter.Crower,Comp,and Crane(a nd Bullitt)do cast iron cores,but make sure you ask.Being a roller the lifters don't rotate,hence you will not need a plasma coating.