No, you don't have to fill it completely to do what you want and even if you filled it completely, it might actually fit you better. My Ballard is no different than yours. Except that is probably a bigger caliber (it's a Pacific .45).
But whatever works for you, works for you. Just suggesting what works for me.
Yours also has the deep crescent Brown buttplate? What's the serial number range? Most I've seen are all 3 digit serial numbers like mine, and were used up in the first less than 1000 Marlin Ballard rifles.