In my opinion and experance a 98 "can" be modified to take any of the cartridges mentioned. However once one goes over the standard magnum length like the Taylor, the .458 and such you run into two big problems. First you in general with most actions have to remove metal hat weakens the action. True the 98 has strength to spare but this is not a good idea all the same. Second if this work is done properly you could buy a magnum length action like a CZ which even has a single set trigger for far less then the Mauser conversion would cost.
The big draw back to using most Mausers action for cartridges .375 length and such is that you have to cut back the lower lug, (the feed ramp) and this and the upper lug recess is where all the strength of the action is and has to be. Any cutting here reduces the strength of the action and your safety. In the case of the Rigby you also have a marginal barrel shank diameter and unless te barrel threads are matched to the action,(They should be in any barreling job), one stands the chance for a buldged chamber if too hot a load is fired as there is little margin for error. Hope I have been of some help, been doing this stuff for years.