S377-19 is not available in half inch increments as a stock item

My SWISCO.com Order included (2) S377-19

However the balances are 0.5 inches too long. Should I get the 18 inches ones and return the 19 inches back to you? will the 18 inches ones work with window properly. I figured the 18 inches ones will accommodate just fine and more likely work, but do not know if the window will open and close properly. I could use the 19 inch ones but will need to drill a hole on the window frame so that the anchor hook from the balance will engage.
What do you recommend.?

Thanks,

Mario

Hello -
I bought a S377-31 : 31" stamped 3040 at the end of the channel. While extending the spring the rivet snapped and plastic pulley cy the hook broke as well.
I am getting another one, but I am concerned if the new one will brake. The other side channel installed fine. If I lubricate the spring or any of the components in the new channel could help? I noticed this channel has a stronger spring than other channels I bought.
Is there a recommended technique or tool I could use to extend the spring?
Your recommendation is appreciated.

thanks,

Mario