The 7-300 Wby followed the 6.5 Wby when long range hunting was born here in PA. One of the leading long range custom rifle
builders was Howard Wolfe, and he specialized in chambering the 7-300 Wby.
The simple solution is to take 300 Wby brass, neck it down to 7mm and be done. No fire forming is needed, and RCBS makes a FL size die and seat die set. The other option is to chamber a tight neck in the range of .312". I have owned both and see no discernible difference between turning necks and not turning necks because the drop in neck dimension does not warrant a must turn neck... I would suggest shooting bullets in the 162-180 grain range, so pick a twist that will allow stabilization, mine were 9 tw. After playing with the 7-300 Wby for a while, I started shooting the 7-300 Wby improved , aka the .284 Baer.
I actually re-chambered a Hart barrel that was formerly chambered in 7-300 Wby. It is extremely accurate with the 180 gr bullets as Chase will tell you....killing PD's just shy of 1k yards in extremely windy conditions. If you choose this route, custom dies are a bit more expensive. I don't fret about fire forming like most of you, or buy into that talk about fire form loads before doing load development... All horse dung...load and start development.
In summary, my personal experience is that the standard 7-300 Wby will suffice with a one step neck down and no neck turning. Plenty of bullets to feed it until you find the right combination.