I don't believe you would have to concern yourself with throating in the 270 Wea as the free bore in all Weatherby's are long enough to allow for all the case capacity to be used. The problem may be in your magazine length . This may restrict your OAL depending on rifle make. You would also need a faster twist than the standard 1/10 to stabalize the 165's. JMO
This, I believe is the correct answer, but I am not sure that it matters...I missed what action you are planning to use.
Anyways...with a stock 270 Wby Mark V action you can only get about 3.38" COAL to fit reliably in a stock mag box.
I am loading my 270 Wby with the 170 Bergers to 3.5" COAL (just got some blem 165 ABLMs to try). So my rounds will not fit in a stock Mark V mag box.
I am pushing the 170 Bergers to 3225fps with RL26 (ES 10 with 10 rounds). I still have plenty of room in the case for more powder. I have "heard" that I can use more RL26 to get more velocity, but I hit a nice accuracy node @ 3225fps and stopped there.
The 270 Wby runs like a .378" freebore, so you should not have to ream a stock chamber.
As another poster stated...you can run 140gr and 130gr bullets past 3550fps and 3600fps with out issues. 150gr bullets you can run them past 3400fps.
All velocities measured with a Magnetospeed chrono.
Shooting a 28" Benchmark barrel, 1-9 twist.
ETA...was able to get the 165gr Matrix bullets to .02" off the lands and heading towards 3300fps. Stopped load development when Matrix went out of business and Bullets.com was blowing out the 170 Bergers for somewhere around $38 for a box of 100 bullets...