Hello again, Cat; once again you are on a productive track.
My barrel is a 28" 1:8" Lothar-Walther
pre-fit.
I believe it will mate up to the Savage Custom Action without any hassle. Yes, the long action would work, but in a single shot configuration, there is no valid basis for it. In fact, there is no need to build an entire new gun; the L-W barrel will turn your 6mm BR into the ideal switch-barrel rifle.
My stock is a McMillan A3, pillar bedded to my barreled action by McMillan. I have a Mueller 8-32x44 side focus 30mm tube scope listed at Amazon coming for $229 with free shipping, I just ordered it today. Although it's lower on the price scale. it has all the features I'd want, gets good reviews, and is listed by Amazon as one of their Best Buy picks.
My brass is made from Win .243, and I decided I'd leave the worrying to someone else, and simply resize it using a set of Hornady New Dimension dies with the elliptical sizer ball, then see what problems may or may not develop. None did. Easy-peasy. Since I don't seat my bullets deep enough to engage any potential donuts I've never seen any issues with or without them.
My load is either 142SMK or 140 A-Max, with 43.8-43.9gr of H-4350, loaded to a length that jumps between .010" and .020". I'm no wind master, but on that proverbial steady and/or calm day the rifle was capable of multiple consecutive X's at 1000yd. Mine is built on a Savage 10FP action, yours should do better.
The folks above are right about the .280 Rem being more authoritative at 1000yd, but it will require a long action, and no matter what you shoot, it could be several years on the 1000yd line before it begins to seem easy. Better to do it with the .260; less expense and less physical stress along the way. The .280 Rem will provide a lot more bore life than a 6.5-.284, the .260's proverbial Nemesis. If the .260 still seems insufficiently authoritative by the time it begins to burn out (I estimate the LW-50 Stainless in my barrel will allow about 3000 rounds before that), then make an
informed decision about upping the ante to the .280 Rem.
For the record, I would have zero qualms about running the .280 Rem in a short
single shot action. The empties will eject from my 10FP action just fine, and any unfired rounds can be ejected by simply disengaging the bolt stop and withdrawing the bolt a bit past it. Again, a properly configured barrel for the .280 could simply be another choice for an existing switch barrel rifle; the 6mm BR, .260 Rem, and .280 Rem all share identical bolt face diameters.
My .280 Rem load is 150gr Nosler BT over 55.0gr of H-4831SC. In a 28" 1:9" barrel I would give it good odds to reach 1Kyd supersonic, the real determinant will be density altitude.
IMHO, there is no better chambering than the .260 to bring a newer 1kyd shooter up to speed.
Greg