Spiral Balance Replacement/Wooden Window

I am replacing the lower sash of a window as part of a bathroom remodel in a first floor addition to my house. The original part of the house is from the 1860s and the extension was built in the early 1950s. I had the lower sash made at the same time I was replacing a lower sash in the original part of the house which has rope and pulley setup. They reused two original windows and put in two multi pane picture windows and one regular sized window on the addition. I did not realize that the reused windows were retrofitted in the fifties with non tilt spiral balances along with the new (in 1950) other wooden windows with thin aluminum tracks on each side (I guess the Anderson or Pella window of their day- I will send exterior shot in separate email).

The new wooden lower sash replacement seems to weigh about 10 pds., according to the bathroom scale and is approximately 28 in high and 32 in wide. The upper sash is stationary and is not being replaced. The existing spiral balance had a flimsy plastic cover on it that is now cracked. The spring part, see pic is a rusty 27 in long.

Please advise if my contractor needs to open a channel all the way down the side of the sash like they did when the retrofitted the old window in 1950. And deep and wide should it be. What can determine whether 3/8 or 9/16 if it a new sash and we probably have create new channel. What is best way to attach the spiral balance to the window and do we have to route out whole length of wind

Hope all this information helps. I look forward to hearing from you at your earliest convenience as this turned out to be and issue when we realized it was not rope and pulley.

Anxious to talk with you. You have a great reputation online for promoting and professional service. Now that I know more about spiral balance I think I will tackle replacement of the spiral balances in the other 1950s windows as the balances are shot and I thought I was going to to replace all the windows.