There are folks who swear by their 605 boxes. The majority are disappointed by the lack of feed back (overpowered) and wandering (no return to center). The 605 steering box is not a recirculating ball design and has a stamped metal cover which controls shaft end play. The CPP500 is a new casting with the internals of a late model steering box. It has great feed back and return to center. I would assume that the new 600 is similar. There is confusion because there was a 600 series box that preceded the 500 series. That 600 series box is no longer made. You can find information on the CPP forum @ ChevyTalk.com
My CPP500 would hold center at speed but would not return to center below 15 MPH with 3/4 degrees of caster. Tri five Chevies by design have limited front alignment adjustment. Adding the CPP tubular upper a-arms allowed 3 1/2 degrees caster. Now the wheel returns to center and the car drives about as good as any new car.
Sorry that your 605 box is not working out. You are not alone. There are other companies that make quality steering upgrade components. I only have experience with the CPP products that are on my '55 so that is all I will comment on.