It's more than just opening the bolt face. The mag is a major deal as the .404 round is fatter than what the Mauser was designed for.
I've seen pictures of Mausers with the sides of the box gone, the rounds running against the sides of the stock inlet, only the rear of the box still there.
Got this on the internet. This is what I'm talking about, the 98 box cut away except the rear wall. The rounds run against the stock inlet. I'm thinking the inlet width would have to be perfect and I'd probably epoxy sheet metal on the sides of the inlet to form the sides, probably the front, of the wider box. I would think that could be tricky if one didn't know the exact dimensions the finished product needed to be. And, that doesn't address feeding issues, follower, etc.
For a slick feeding 404J on a 98 action, you'll need a real pro to tackle this type project, IMO.
As noted, I built a .404J on a Classic NH M-70 RUM donor. But, IMO, a .404J on a std 98 is another matter. I wouldn't tackle it.
DF