I would never buy a base stock looking to change out rifle barrel calibers with the expectation of an entire rifle/scope being sighted in to where I was absolutely confident of the entire rifle & scope's accuracy with several hundred rounds through it at the range I was planning to hunt at.
On my new 300 WINMAg I purchased 18 months ago, I put 200-300 rounds through it between 100-200 yards. Two months before my trip, I starting seeing a 1.5" - 3.0" variance out of every 6-10 shots from the normal. Drove me nuts as I could not isolate until i found one scope mount screw that was maybe a 1/16th turn off.
I have multiple rifles ... 30-06, 308, .50 cal TC muzzle, etc. All are very good. Can't believe how flat shooting my Ruger Hawkeye 300 WIN Mag is out to 200 yards. No ballistics guy here, just want a rifle that I can be very confident in challenging hunting enviroments. Plan on pushing out to 400 yards this winter.
For what you are talking about Jack ... you could do 2 rifles, a 270 & 300 WINMAG and cover north american and even Africa.
Have Leopold & Nikon scopes, consider Vortex ... former guys with Zeiss I belive, I know have 2 of them, very impressed.