The Mountain Tactical barrels are not "screw on", they have to be fitted.

$940 for a Proof Research barrel is an expensive gamble for my tastes.  For that cost I can't understand why anyone would avoid having the barrel fitted to the action, plus a check over of the action and bolt to square it up as needed.  I would buy a gun first.

In this case the bolt face is the same for both cartridges and the length of the .270 Win is longer, so that conversion is possible without other complications.

Any cartridge with a rim diameter of 0.473 inches will fit the bolt face.  The action is a "mid" length between typical short and long actions.  I think the best way to find out which cartridges can used is by looking at the list of cartridges available in the rifle and make a list of their overall lengths.  The magazines are shimmed to accommodate different cartridges, so you would want to buy magazines for the specific cartridge and understand that it might need custom work.

I'm closed minded about a pre-fit barrel.  Maybe shouldn't be if the supplier can supply barrels that head space.  That means they have enough dimensional tolerance information about the action to produce a barrel shank that fits to a headspace gauge.  Modern geometric dimensioning and tolerancing along with three dimensional model based definition allows that if the producer has the information and manufactures the barrel to those dimensions.  I just can't being skeptical that it's the right way to go.  A barrel nut removes that hesitation, but doesn't remove the need to at minimum square the action face.

I have a SAKO A7 that shoots like a custom rifle, and if they all shot that way I wouldn't ever look at a custom gun again.  I expect SAKO and Tikka are assembling rifles with actions and barrels brought together at the bench without custom fitting, although I'd also bet any assembly that didn't headspace correctly is set aside for rework before going out.

I'm just not wired to spend several hundred dollars and hope it works out, I'd rather take the path that insures it does.  I also won't deny that if it works, then it works.  I'll be skeptical until then.