Re: long throat in 300 mag
If your rifle is a repeater, I would load to the magazine length and not worry about how far your bullets are from the lands. There's not much you can do about it. Shoot it at that length, and see how good the accuracy is before you worry about seating into or just off the lands.
Once you have an accurate load you can tune your rifle to shoot that load even more accurately.
If you are going to shoot it single shot, seat them out as far as the bottom of the neck, this will give ample bullet pull for single loading.
All Weatherby rifles/cartridges have 'freebore' and they are very accurate with tuned loads and rifles.
The only other option is to re-barrel with a 'match' chamber cut into it.