Re: A bunch of simple questions.
The 338 is the best way to go, as wind will be the biggest consideration at that range. Look at the BC's on the available bullets and what speeds you can expact to compare the choices.
The 338/378 Weatherby cartridge is the simplest, all out performer there is. If you buy custom dies, the 338 Lapua Improved is right there too, and with better, longer lasting brass. Standard 338 Lapua = next best performance. The 338 Ultra is a close fourth, with even cheaper brass yet, not the quality of the Lapua, or life span, but initially cheaper. You're smith may not be willing to open up the bolt face on a M70 or 700 for the big Weatherby or Lapua case, so unless you buy a custom action or have GA Precision or another smith make the modifications and chamber for it for one, the Ultra may be what you're left with.
All brass is available.
Custom dies run about $275-350
Custom Reamer $135
Ackley Improved versions should fire form from factory brass.
G5 should correctly model the 168 J4, just compare that the the drops you verify at the range to know for sure, G7 or G1 for what ever reason "may" model is better. Bullets advertised G1 BC must have the BC converted for the true BC of each drag model. Many ballistic programs have a converter, if yours don't, email me and I'll convert it for you.