Glad you asked, I wish I would have!
I am dealing with a lemon (perse`)of a Model 70 Winchester. It is an older Model 70 XTR originally chambered in a 7mm Rem Mag then re-chambered to a 7 mm Shooting Times Westerner and laid in a Ramline synthetic stock. My uncle was a major gun collector and was ridding himself of several guns, including this one. I was in need of a LR 7mm and he said that if I could get this one to shoot well, he'd just give it to me. Well, I did. Ended up getting 3/8 5 shot groups. However, it was a single shot. When I reloaded for optimum cartridge length which was right at SAAMI specifications the cartridges would not fit in the magazine. Sure, I could seat the bullets deep and get it to work but my accuracy would go out the door. So, I am having a gunsmith look at it. He said that if it was a Remington action, the re-chambering would have been all that was required. However, be that as it may, he's looking into doing some magazine work to get the cartridges to fit. But in all honesty, it looks like I'm going to have to re-barrel.
He's been gone for some time and hasn't had a chance to get back with me. In the meantime, I've been able to do some research. As far as re-barrelling goes, I THINK I can simply change to the following calibers without any further modifications: 7mm Rem. Mag (of course), 300 Win. Mag. and 338 Win. Mag. I believe these will work based off the fact that they are all based off the same parent cartridge (300 H & H, I THINK). Whereas the 7mm STW is based off of a 8 mm Rem. Mag (in turn based off the 375 H & H mag). I think if you do some digging, you can find out which cartridges can be used. A good place to start is the website below:
Chuck Hawks Rifle Cartridges Home Page
He doesn't have many wildcats like the 7-300 Weatherby so you may have to do some digging elsewhere.
By the way, there are plenty in here that will attest to the 7mm Rem. Mag's Long Range ability, even out to 1000 yards. So, unless you are just dead set on getting a new caliber, you may want to stick with the 7mm Rem Mag you have, as is.
Good luck and hope this helps.