S850 series crossbow balance

all of my larger windows have broken/missing balances. I can identify that they appear to be crossbow balances by the parts that I have. my question is this: can you determine the length of the balance you need by the size of the window? my large window is 37"W x 58"H. From watching the replacement video it appeared the sash started about half way up the bottom window to the top screw mount. I ordered two 36" 850 balances. last question: If the balances are a bit short/long can you drill a new hole in the top to adjust the position?