Thanks for any help with this and feel free to e-mail me for anything else you need to know to identify which set of window balances I would need to purchase

Thank you for posting! A Series 390 with a 40" channel and a 3960 stamp option will have to be custom made. With that said, I'll email you a purchase link that you can use to place an order on our website.
A S390-40 with a 3960 stamp option will carry a window sash weight range of 35 through and including 39 pounds.
This information is based on 2 balances per sash. Balances are designed assuming friction in the jamb weather-strip will carry a portion of the sash weight. Suggested sash weights are approximate.