Your choice of the 270 for antelope is a good one. I shot several with mine all with 130 gr Nosler Ballistic tips. But, I am assuming you don't handload.
With that in mind, I would take your rifle out to the range witha few different130 - 140 gr factory loadings and see which ones shoot the best for you.
If this is your first antelope hunt - I think you are going to enjoy it. They are really fun hunts. And 3 days is plenty of time to take a good one if you are in a good area. Shots in the 200 - 300 yard range is pretty much the norm. I believe if you sight your 270 in for 3" high at 100 and you will be good out to about 325 or 350. I don't recall anymore, I use the Burris ballistic-plex scope on mine and it works like a charm.