Hi, the mag box on the howa is the same as the old Sako Finnbear- 91-92mm long. You would need to seat bullets to 90mm for smooth feeding. Bullet jump will be up to 12mm (.5") which would make even old Roy WBY turn in his grave. Anyway, you can work all this out for yourself as the .300 WBY ammo you have is near identical in both case length and COAL as the STW.
You have a good solid receiver so don't get rid of it but (sigh) you need to go back to the drawing board and have a re-think. Looks like you might be in for a wildcat. I am thinking about the same for my M70 winny. Am preparing to build a 7mm-300 Win mag improved. Velocity will be the same as the STW and RUM with 160gr bullets, slightly slower with 180 grain bullets. There is only so much energy you can force through a 7mm Bore and this wildcat is as good as it gets if throat erosion is to be avoided.
160gr- 3275fps
180gr-3050fps (conservative)
Its a good cartridge for all of us out in the wop wops. Brass is easy to obtain, the case can be fireformed with 'goat loads' and it won't create too much of a headache for your gunsmith as he can use a fresh 7mmRM case belt to headspace the wildcat. This puts it ahead of the Dakota etc by a long shot.
If you are planning a trip to NZ at any time, there are folk who can have your rifle re-barreled pronto. You would need a reamer from PT&G (google PTG reamer) and they will already have details of Aussie smiths on file so should be able to point you in the right direction.
OK- will try and re-do the post in Australian language (my old lady is an Australian/ Gold coast)
Hey Siggo- your pullin the raw prawn if ya think that japo action is gonna take that thousand mile bindie buster cartridge.
Have a beer.
What you need is a wild dingo with a firecracker up its a'''s. It a good number for us back country yobbo's.
Its a fair suck of the sav that you can't get the job done here in NZ. Get ya misses to google PTG + reamer after she gets you another beer.