Whilst it is possible to build a 416 Rigby on a standard length action it is not considered best practice for the following reasons.

It reqires removal of not insignificant ammounts of material fron the rear of the bottom bolt lug.Which compromises the strength of the action.
It requires removal of material from the bolt stop and contributes to "bolt wobble" and possible bind.
I requires a complete rebuild or replacment of the mag bottom metal.
Often it requires a cut in the rear top of the reciever ring again a strength issue.

The strength issue was less important when "handloading" was relitivly uncommon, as the manufactures could be reasonably certain that they could determine the stresses to which their rifle would be subjected. In the modern age of litigation and handloaders, for a company to compromise the strength of an origional design and then sell it to the public, would be a recipe for disaster. I cannot think of a respectable company that would do it.

It is not cost effective. If one employs a gunmaker suficiently talented to carry out the work correctly, the cost will exceed that of purchasing a 98 Magnum action of the correct size.

The best 416 for a standard 98 is the 416 Taylor which deserves I think to be far more popular than it is and to be factory "legitimised".

Regards