I just re-barreled my 300 Wthby Mark V Alaskan to 300 PRC. The Weatherby barrel was too thin a profile and would walk all over the place (and it ain't the shooter I can guarantee you on that ;-). I worked with Carbon6 and went with a 26" in bull profile...barrel is super light and I've not had a chance to do load development yet...still working on getting into a McMillan A6 stock before shooting as the factory stock is way too narrow in the front end to accommodate the larger diameter Carbon barrel....there would be nothing left 4" forward of the action.
The folks down at Carbon6 were fantastic to work with - and I am ecstatic about the final product. They do have narrower profiles as options that would be insanely light weight and they still guarantee 1/2 MOA. Other than Carbon6, I'd recommend McGowen as they will make to your specifications and will even install your barrel if you'd like. I've heard fantastic results from McGowen barrels/re-barrel jobs and our local top gunsmith uses them for anything he doesn't have an interest or time to build from scratch himself...he has really high praises for their quality.
On Calibers, I'd depend really on final intent...if competitive or hunitng mostly deer with maybe an Elk thrown in, then I'd personally go with the 6.5 PRC (or 7 SAUM if you can find brass) for reduced powder and excellent ballistics. If for hunting Elk or larger, you may consider the 7 PRC (easier to find factory ammo in a pinch) as it maintains excellent ballistics as well with the ease of finding brass and factory ammo.
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Take care, good luck with your project, and God Bless!