I am going to buck the trend here and suggest you build a 270 WSM rather than a 300 WSM.
The reason for my suggestion is that the WSM case does not have sufficient case capacity to provide enough muzzle velocity to optimize the performance of the optimal 30 caliber bullet (approximately 230 grains) relative to the optimal 270 caliber bullet (approximately 170 grains). Assume that you built two rifles (one 300 WSM and the second 270 WSM) on that same action that were equally accurate and you tested both builds with the optimum .308 and .277 bullets using loads that you developed that achieved the highest MV in each case, there is a higher likelihood that you will hit your intended target with the 270 caliber bullet more frequently than you will with the same action with a 30 caliber bullet (at long range).
I own several 300 RUM's, .308's as well as developed loads for 300 WIN Mags owned by several of my friends. I built two 270 WSMs for my twin daughters and developed loads for those rifles as well. All the bolt action rifles within my family are sub ½ MOA.
I live in Utah and we use our rifles to hunt deer, elk and antelope as well as to shoot long range at steel plates in the desert west of Salt Lake (I use my 300 RUM hunting rifles out to 1 mile + and the 270 WSMs to 1000 meters +). We've used both the 30 and 270 calibers to take all three species (all animals died quickly) so they work just fine at longer ranges (out to 800 to 1000 yards).
The reason for my suggestion is that I suggest you build the rifle around the optimum bullet for each caliber you are considering and that you match the bullet weight to the case capacity so you can achieve the optimal MV and downrange performance (more hits on the target). If you read Bryan Litz' books, he has at least one chapter on the optimal bullet weight for a given caliber. He has also written articles on the 30 caliber bullet and why it is less popular as a "long range bullet" than other calibers (the same problem existed with the 270 caliber until recently) – Please see Litz' article "what is wrong with .30 caliber?"
http://www.appliedballisticsllc.com/Articles/ABDOC110_WhatWrong30Cal.pdf
The short answer is that there is nothing wrong with the 30 caliber or the 270 if you match the optimal bullet weight to the caliber (i.e. the diameter). There is an optimum bullet weight for each caliber. Specifically, the optimal weight of the bullet is sufficiently high enough to maximize cross sectional density (and BC) without increasing the length of the bullet (to achieve the greater weight) so much to the point where it become too difficult to stabilize the longer projectile. The optimal weight for the 30 caliber is around 230 grains and the optimum weight for the 270 is around 170 grains. Please read his article. I don't have the technical expertise to debate the topic nor do I have the empirical evidence Bryan has collected to substantiate the claim. I can say that my rifles 30 caliber and 270 caliber rifles perform better with the optimally weighted bullets for each caliber than they do with other bullets within the same caliber.
To optimize the performance of any given caliber, I like to obtain a reasonably high muzzle velocity for the bullet I am using (2900 fps to 3100 fps) while simultaneously maintaining accuracy and staying within recommended pressure limits. When I developed loads for the 300 WIN Mag, the heaviest bullet I used was a 210 grain Berger VLD Hunter. The muzzle velocity I achieved with these rifles was between 2700 fps and 2800 fps. When I used a 230-grain bullet (the Berger 230 grain Hybrid Target) in the 300 WIN Mag, my MV dropped by 50 fps to 75 fps (while still maintaining accuracy).
In all my 300 RUM builds and 270 WSM builds I achieve MVs of around 2950 fps to 3050 fps using the optimum bullet weights (230 grain for the 308 caliber and 170 grain for the 277 caliber). Applied Ballistics has software that simulates the distribution of hits (aka the groupings) on the target given initial conditions (e.g. specific bullet, MV, group size at 100 yards, atmospheric conditions, distance to target, shape of target etc). You can use that software to test the performance of your intended rifle builds without going through the expense of building multiple rifles. That is a great deal!
When I simulate the same bullet (e.g. the Berger 30 caliber 210 grain VLD) fired from different rifles under the same initial conditions accept one rifle produces a MV that is 250 fps slower, the probability that I hit my target at distances beyond 500 yards decreases significantly using the rifle with lower MV. Similarly, if I use the simulation to compare the Berger 30 caliber 210 grain VLD at 2700 fps vs the Berger 270 caliber 170 grain Elite Hunter at 3000 fps, I have a higher percentage of bullets landing on my intended target if I use the 270 caliber bullet (traveling at a higher MV) than the 210 grain bullet. In his books, Litz runs comparisons between the Berger 30 caliber 230 grain Hybrid Target and several brands of 300 grain 338 bullets (Sierra, Lapua and Berger) and the 230 grain Hybrid Target outperforms at least one of the other brands as measured by hits (distances beyond 1200 yards).
The point is that the optimally weighted and shaped bullet for each caliber can fly materially better than the other bullets within the same caliber or even across calibers (e.g. Litz' test of the 230 grain 30 caliber bullet vs. 300 grain 338 caliber bullets) which will result in more bullets hitting your intended target (particularly at longer ranges). Each case has only so much propellant capacity and the heavier the bullet you use for a given case will result in lower muzzle velocities for that given case (assuming you don't want to exceed maximum chamber pressures). If I wanted to build a 30 caliber rifle for hunting, I would build it around a larger case capacity than the WSM case to maximize the performance of the optimal bullet. If I was given an action that could be used for either a 270 WSM or a 300 WSM, I would build a rifle around the bullet and seek to optimize the performance of the bullet. I would build a 270 WSM.
I realize that I did not address the issue of "downrange energy" which is relevant to hunting. My theory on that topic is that downrange energy less relevant if the trade of is a reduction in the probability that you hit your intended target. In addition, my experience with both the 270 WSM and the 300 RUM had plenty of downrange energy to kill all the animals we were shooting at.
Sorry for the long answer. I have had this debate with my friends for a long time. I have also spent 000's of dollars building rifles only to have "case capacity envy". Specifically, I would build a new rifle only to learn that the case that I built my rifle around was insufficient to launch the optimum bullet for the caliber of the barrel. I would then have to switch out the barrel or sell the rifle. I now start my rifle builds with "what caliber do I want, what is the optimal bullet weight for that caliber and can I use the receiver with the case capacity that will produce the maximum MV for the optimal bullet in that caliber.
