Electrical 1991 5.0 battery size

If this is normal, I am thinking about cutting two pvc strips from brick mold or other trim and gluing them into the battery box to reduce the chance of the battery sliding front to rear if the clamp gets a little loose. The box looks just like what I remember in my 79, just on the other side.
If a group 58 has plenty of CCA, putting in a bigger battery seems like a waste of weight. But I do not want a battery sliding and cracking the tray or hurting the overflow tank lid if I go auto crossing or driving enthusiasticly.