If they're comparable sizes, I don't think radials vs bias-ply makes any difference. Now if you want to put on larger tires (either kind), it's another story. If you can't get an answer, do what we did when we were kids. Jack the car up and remove the existing shocks and see if the rear end drops any more. If it does, you have your "long" measurement. (Stock is the "short" measurement.) We used to go to the junkyard, "exercise" a few shocks and make our pick. Nowadays you have the advantage of charts and specifications on the the internet.
I know which I prefer.