What, exactly, is it you are trying to accomplish with your build? I am a huge 6.5 fan, but with your stated requirements, you are going to be taking a step DOWN from where you are with your 7mm Rem Mag.
If barrel life is a priority to you, you are going the WRONG way. When you reduce bore diameter, you will also reduce barrel life, unless you go with a smaller case. If you do that, you will be giving up a LOT of performance vs your 7mm Rem Mag. Going to the trouble of building a custom rifle to reduce your performance level doesn't make much sense in this scenario.
You could go with something on the order of a 6.5 SAUM, which will give decent barrel life if you don't lean on the throttle. But, that is still taking the long way around to get performance that will not be a step up from your current rifle. Unless there is something you don't like about your 7mm Rem Mag, why not simply build it up?