I can save you money and time by borrowing my savage action wrench and barrel nut wrench (small/standard size, not large). The 2 didn't cost much and I have headspace gauges. Money saved all around and all you need is a barrel (and maybe a barrel nut that's around 25-35 dollars). Also, a recoil lug. I've bought barrels for savage 110 actions for builds I've done and added lug and nut to the custom barrel order (McGowen/C6). As noted before, check northland shooters supply as he's great person to buy from and deal with. Headspacing is easy and not an issue to do. Going shouldered prefit will need more info before buying as so it goes on pre-headspaced. I've not done 1 before, but requiring a measurement from the bolt face to the front of the action (I believe) is what is taken? @bamban talked me thru it. If the prefit is saami chambered factory ammo should chamber fine? Going from 243win to 6creed isn't much of a change. Creed has higher max pressure, but lower case capacity which about equals 243win speed with lower pressure, but higher case volume. As you stated it's for longer boolits? Well, if using the same mag and restricted to 2.800-2.830" then it does let you seat them out further as the case design is for. I chose my 243win over 6creed and 260rem over 6.5creed (not counting the 5 6.5creeds's I have).