You can cut down a longer bladeif you ar taking less than 1" off each end and you balance the blade when finished.
Most of the aftermarket blade suppliers have tables of blades by length
I just cut down a 30" blade to fit a 30" murry where the blades are marked as NLA
When you cut the end off you have to cut it back at a small angle so the end is not square and the cutting tip is around 85 to 88 deg not 90 deg