In accordance to what others are saying; to shoot a 338 Lapua, the same as your dad in order to compare apples to apples, I'll say this:
Shoot another factory rifle.
Your dad is shooting a factory Savage 110 BA, so you shouldn't get to build one custom.
You still want your caliber to be "able" to outbest his though, I get that. So I'm here to put another vote in for the .338-378 wby.
Weatherby makes that caliber in the Mark V. That's the best starting point I come up with.
Happy shootin! Fill us in on what you go with.