We're a bit off the topic of shocks, but...

No radials weren't around then, which is why the poly glass design was such a decent performer. Widespread use of automotive radials didn't happen until the mid to late 70s. Old timers will remember why Pontiac was heavily advertising their RTS line and that wasn't Rapid Transit System but their Radial Tuned Suspension. All it amounted to was more agressive alignment specs and better shocks, but it sure threw a lot of consumers, and tire dealers for that matter, for a loop.

Bias ply tires will perform decently. A comparable radial will be faster once the systems is set up for it an the driver gets used to it. But a bias ply is also more forgiving of a tire for driver inexperience when run at the edge of grip. In my experience, bias plys tend to "broadcast" their loss of grip in a more linear and predicatable manner. Similarly, when traction is lost they will regain it in a similar linear way. Radials, on the other hand, tend to break loose more suddenly and not regain traction as quickly.

Thinking back to my oval track days, it took us several weeks of experimenting when we swapped from bias plys to radials before we were able to get the laps speeds back up. But once we did, we also started running faster too.

BTW, the f.a.s.t. cars are running plenty fast on bias plys. Stock reproduction bias play no less. Because of what I said above, a number of drag tires are still available in bias ply as well.