I am not sure if there will be any problems. I don't think so, you hear of people building 6.5/284 on LA without any problems. Talk to the gunsmith you are thinking about using.
Milling out the back of the short action and fitting an extended magazine box is not a big job if you already have a SA, although if it is a Rem Model 7 there probably isn't enough metal at the back of the magazine to do it. A Rem Model 700 won't be a problem.
Stu.