Re: 40 MOA picatany rail
US Optics will make you a custom base. But there are plenty of one piece scope mounts with lots of MOA built in, also some scopes can be had setup like that. So stack them on a standard rail & you'll have the reach you need.
What do you plan to use as a spotter at that distance, the Hubble? I mean to see bullet impacts. Might need some good thermal gear with bullet track radar, as if there's any haze, or dust in the air, or mirage, much over 12x ain't gong to be useable.