Issue with 124/127 guns is that the spring guide is integral to the rear block. Some springs fit better than others, tho currently ARH is advertising a snug replacement spring for these guns
The Pro-mac slightly softer kit is the one I would have recommended. Easy installation and a very smooth upgrade for the ol' girl. Great to see you revive that 124. You'll definitely enjoy the fruit of your labor of love restoring the gun to its former glory!