Retrofitting historic wooden sashes with spiral balances

I need to replace several window frames but preserve the wooden sashes because they are in one of several condos in a historic building. The upper sashes will be permanently fixed in position and I intend to fit the lower sashes with spiral balances.The windows are grooved already, so I can route them out further to accept the balance instead of grooving the new frames. The sashes are 27' and weigh 18lbs, can you tell me what balances and additional hardware I'll need. I understand that once purchased I can't return them but there is already one window fitted with a spiral balance in this unit so I am reasonably sure this scheme will work.

Thanks, Pat

Thanks for replying. The sashes I'm working with do not currently have spiral balances. At some point they were retrofitted into an ill fitting PVC Casing with a coil balance, all of which have failed and must be replaced. My intent is to remove the entire existing frame, permanently secure the upper sash, and retrofit the bottom sash with a spiral sash balance.

I can modify the groove to take either3/8' or 5/8'.

They weigh 18-20 lbs, and I would obviously prefer to over compensate if possible.

The Sashes are just shy 0f 28" X 34 5/8" and the over all finish height of the frame is 54", so the distance from shut to open is 26".

My main point of confusion is what clips and stops I need.