I would also go with a Long action make sure you DONT use the 115gr Bergers you will blow them up the 107gr Sierra or 115gr DTAC would be the best options with the 115gr as a great starting point in an 8 twist barrel.
I agree on using the long action but I have a 6/284 on a short action and it works fine. The difference being in the twist, mine is a 12" so I am shooting the smaller/shorter bullets (55-85). And I able to get these in a detachable magazine.
I have one built on a short action. It is a 2 shot repeater the way the mag well and the box is cut out for the spring and follower I am allowed one shell to fit in it. If it must be a repeater then i would do a long action or opt for spending extra cash for a wyatts box mag.