You have two options fo putting any stock made for an AR to a 556.
1. Use the early version 556 that was made to take the AR buffer tuber. I am sure you would have no problem finding someone with an old lower to swap for your classic lower. You would need a front pin for the old lower to replace the screw pin if it still has those.
2. DDLES makes an Adapter to put an AR stock on a 556 classic lower, but it isn't released yet.
1. Use the early version 556 that was made to take the AR buffer tuber. I am sure you would have no problem finding someone with an old lower to swap for your classic lower. You would need a front pin for the old lower to replace the screw pin if it still has those.
2. DDLES makes an Adapter to put an AR stock on a 556 classic lower, but it isn't released yet.