The 98 third bolt lug is a safety lug. It doesn't contact the recv'r, it sits a very few .000 clearance fit away.
It is there to hold the bolt should the front 2 lugs set back or fail.
The Swede Mauser never had this. None of the Small Ring pre-M98 Mausers had a 3rd/Safety lug except one of the Model 95 Mausers which used the root of the bolt handle sitting in a notch in the recv'r for the same reason.
One of the 1960 Husqvarna commercial production M96 'Swedish Mauser rifles was updated and included the 3rd safety lug on the bolt.
Also the gas flange on the bolt shroud, another piece of safety equipment that the Small Ring actions don't have incl the Swede Mausers.
But the S/Ring Mausers make wonderful rifles. Anyone of them can be made into a fine sporter and the Swedish Mauser action is usually the top pick of the S/R actions.
Keep the caliber & it's loads within reason for the action design and you'll have no issues with the rifle.
Try and push it beyond it's limits and you can expect it to do things like 'set back' and even worse. They are not made to handle hyper pressure calibers and loads.
The orig caliber is a great all around one and actually needs no substitute. But there are some other fine rounds that will also fit in just as well in the action and not put any strain on it. 7x57Mauser is a excellent choice for a S/R Mauser.
Here's a eye-candy beauty of a 6.5 Swede caliber sporter on a Model98 action built by Dorleac in France.
Forums.NitroExpress.com
They have used the 96 Swedish action for custom builds as well..