if your having a new barrel chambered build a dummy round to the dimensions you like and have it throated for that round, as far as loads go I cant help you
Because of the lenght of those looooong 264 bullets you will be limited by the length of your action and magazine.
I have a 6.5 rem mag but it was a rebarrel of a 300 win mag (long action) and I stick them pretty far out and could go further
If you have a Remington or Winchester it is possible that you could get a Wyatt's box and increase the length of your magazine but if it is a short action you will be limited by the ejection port and may have a feeding issue if you make the loads too long.