I have done two myself and I know several smiths who have done quite a few of them. Two of them are members here on this board. The reason they refuse to do this work any more is because of actions that failed after someone loaded stupid. That is the fly in the ointment and the reason it is so hard to find anyone to actually do the work.
I certainly understand that, because a smith has no control over how hot some idiot loads the ammo, yet is liable if the something happens. As has already been posted, Parker-Hale, Browning, FN, Interarms Mark X, Zastava and a host of others use the standard length Mauser for the 375 H&H, but they have deeper pockets than any gunsmith I know.
That is something you will have to remember if you do the work, and eventually sell the rifle. The problem is that the conversion is fine for factory ammo load levels, but marginal for hotter stuff, notwithstanding the fact that years ago Weatherby used Mausers for his 300. There is a guy up here who has an original 300 Wby on a 98 that he has owned since the late fifties. He hunts with it every year and shoots nothing but factory ammo.
Building 375s, 300s, 425s, 500s and other long and huge stuff on standard Mausers used to be done all the time. Harry Selby's 416 Rigby was built on a standard military 98 with the thumb cut-out on the side rail. It was built by Rigby themselves. Selby actually wore out the first barrel and had another installed, so it will certainly work if the pressures are kept sensible, in the low 40s like factory ammo.
Unfortunately, once lawyers got involved most gunsmiths stopped doing it. Even if you prove it was the loads and win in court, it cost a zillion to defend oneself, and you never get rid of the sullied reputation.
Ted