I'd highly suggest either doing a Cerrosafe cast of the chamber or using a pin gauge to determine what diameter the chamber neck is. For whatever reason, even supposed 'no-turn' 250 Ackley reamers seem to run the gamut for neck dimension...from .284 to .288. Much of this goes back to the parent brass that was around when the reamers were spec'd. Make sure
what you have. Since the barrel is off, it's simple to do. You may want to have the 'smith doing the fitting do this before
it gets fitted.
Once you know what the chamber neck is, seat a bullet in the brass you're going to use and check the neck O.D. with a micrometer (not a dial caliper) to determine you've got adequate clearance.
Make sure the 'smith that does the install is experienced with how to headspace an improved cartridge. Traditionally, an 'improved' chamber will result in a new, un-formed case being .002-.004 'tight' when the bolt is closed. This assures positive headspace when the case blows out during f-forming. There are other ways to accomplish this as well...but this is the 'standard practice' method.
An improved case like this is one of the best and easiest to work with. Rather than a true 'wildcat', it's what I like to call a 'domesticat'.
For whatever reason, mine likes powders on the faster side of the spectrum. My standard dual-purpose load is the Sierra 87 gr. flat base over H322. That bullet works perfectly on everything from coyotes to white tails to antelope. For mule deer, the Nosler 100 gr. BTip always works. Winchester 760 and Re15 are good powders with the heavier bullets.
Hope this helps...be happy to walk you through anything else you may on this project.
Good shootin' -Al