Newbie, may have bit off more then he can chew

I'm the newb, I've been lurking here for awhile and decided that I want to re-barrel my 300 wm into a 280 Ackley improved. The 300 is a Savage Model 111 and the new barrel will be a McGowen. I've already ordered it. They needed to know what I wanted, so it ended up being a semi custom order. As I was finding out the shank size I found I have one of the smooth barrel nuts. This started a whole new search on how to remove those! Anyway after I ordered the barrel I came across a little diagram of different action sizes. Now I can't find anywhere where it tells me if the two cases will work in the same action. Anybody here able to help me on this. If I screwed up then what action size am I going need.the diagram showed that you measure the opening of the action. Mine measured 3 3/4" All this research is kind of fun and habit forming. But my first attempt might turn out to be costly. Thanks

Both rounds will work in your action. You will need a standard long action bolt head (.473 bolt face). Everything else should be gtg.
ETA: buy a new barrel nut preferably with ribs for your screwing pleasure, and a 280AI go gauge too.

You are on the right track. Need a barrel vise or an action wrench to secure then use a pipe wrench to remove the smooth barrel nut and use it as a paper weight. I very easily made an action vise out of a 6 inch piece 4x4 block of hickory. It was very easy to do with a drill press, hole saw, table saw, a couple 3/4 in bolts/washers/nuts and a dremel tool. The action clamps in the block and the block clamps in a regular vise.

Talk to Jim at Northland supply1-763-682-4296 he will get you headed in the right direction.