I hate talking people out of one caliber or another, so in fairness:
I don't know how much talking out of one caliber for another is being done here. Just that the OP listed some chamberings that don't have factory rifles being built to go long range. The closest is the .243 Win, there are a lot of good bullets out there for the 6mm and it is a strong contender at the long range game. When I went and built my LR rifle I chose the .243 as well.
I used the Stevens 200 for the basis of my build. I chose a 1:7 twist McGowen varmint contour 26" barrel so I could run the 115 grain DTAC or Berger bullets if I wanted. I just don't quite have the optics I want yet but right now I'm running a 3.5-10X40 Weaver GS Tactical on an EGW 20 moa base. I added as well a SSS competition trigger, Stockaded recoil lug and tactical bolt handle.
Hindsight, I think I would have liked to do the Creedmore and I may yet. I just haven't had a chance to shoot my rifle since I got it put together. If I really get serious about this LR stuff though I'm sure I'll step up my next rifle to a full custom.
That said I'm hoping my accuracy is there for more than 800 rounds, I figure I'll shoot up at least 200 in barrel break in and load development. I'd hate to loose 1/4 of my usable accuracy without ever getting to shoot it at the Sporting Rifle Match in Raton NM. Of course it depends on the type of shooting the OP will be doing as to what kind of accuracy he needs. Me I'm just hoping for 1 moa or better as the match I want to do you only get scored on hits on targets ranging up to around 2 moa in size out to almost 1000 yards. It consists of six steel targets per station, ten stations total, one shot per target, max score of 60 possible.