170 grains is an excellent weight for that cartridge. Get an LCFN and the body will be mostly contained in the case neck.
To determine the best diameter. Measure the outside diameter of a loaded case neck and that of one fired with a full load. Whatever the difference is, add to the diameter of the bullet which the case was loaded with. That will be a good sizer diameter, and order your bullet the same diameter. If the chamber neck is greatly oversize I may tighten things up a bit so gas checks will fit, and perhaps make the first driving band at groove diameter if you know what that is. But we'll cross that bridge when the order gets here.
