Custom Channel balance request

Hello. I'm looking to replace the two channel balances on a non-tilt, vinyl single hung window with thermo-pane glass in my home. The sash size is 17 1/2" x 49 1/4" and weighs 21.2 lbs. The metal channel width is 9/16 and length is 15". The bottom nylon shoe depth is 1". The stamp on the channel balance is 1490 21. From your part finder guide, it looks like I would need an S351 channel with a 15-002 bottom attachment. I'm assuming since my sash weighs 21.2 lbs the strength needed will exceed the 1460's strength limit of 20 lbs and may need to be custom made. Please advise whether my assumptions above are correct and if so provide a quote for two balances or let me know what else you need to know.