Hmmm- good points.
My 270 is set up with 140 gr. TSX at about 3050 fps and my 300 Win mag is set up with 180 gr. Accubonds at about 3050 fps.
I haven't shot higher grain bullets than those two before and my max range at the range has been at our 500 yard marker. The 300 win mag seems to be more accurate and drop less than my 270 at that range.
Should I consider anything more fancy than the cartriges that you list, like a 6.5x68 or a wildcat or just hunker down with one of my rifles and see where I end up at the end of the summer?
Given your time constraints, I'd use your .270, order a bunch of 165gr Matrix VLD's, mount a scope that has repeatable, reliable turrets, and get to practising. Your .300WM will likely get pushed around in the wind a bit more, but if you want a quick and easy solution, just start practising with those 180gr AB out to 900 yards. The key word in all of this being "practice". Better bullets will increase the margin of error in your wind calls, but practice is going to be the biggest deciding factor in your ability to pull off a 750 yard shot on an animal.
If you had more time, you could get as fancy as you wanted with the chambering. They all work. The bullets, rifle setup, and optics are what make the difference.
